High-density acicular hematite particles, non-magnetic undercoat layer and magnetic recording medium

ABSTRACT

High-density acicular hematite particles comprise acicular hematite particles and a coat comprising an oxide of tin or oxides of tin and antimony, formed on at least a part of surfaces of said acicular hematite particles; and have an average major axial diameter of not more than 0.3 μm, a pH value of not less than 8, a soluble sodium salt content of not more than 300 ppm, calculated as Na, and a soluble sulfate content of not more than 150 ppm, calculated as SO 4 . Such high-density acicular hematite particles is suitable as non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium using magnetic particles containing iron as a main ingredient, have an excellent dispersibility in vehicle and a low volume resistivity.

This is a division of application Ser. No. 09/048,179, filed Mar. 26,1998, is now U.S. Pat. No. 6,207,279 the entire content of which ishereby incorporated by reference in this application.

BACKGROUND OF THE INVENTION

The present invention relates to high-density acicular hematiteparticles, a non-magnetic undercoat layer containing the high-densityacicular hematite particles and a magnetic recording medium having thenon-magnetic undercoat layer. More particularly, the present inventionrelates to high-density acicular hematite particles suitable asnon-magnetic particles for a non-magnetic undercoat layer of a magneticrecording medium using magnetic particles containing iron as a mainingredient, which have an excellent dispersibility in binder resin, a pHvalue of not less than 8, a less content of soluble sodium salts andsoluble sulfates, and a high surface conductivity; a non-magneticundercoat layer containing the high-density acicular hematite particlesand suitably used for a magnetic recording medium using magneticparticles containing iron as a main ingredient; and a magnetic recordingmedium having the non-magnetic undercoat layer.

With a development of miniaturized and lightweight video or audiomagnetic recording and reproducing apparatuses for long-time recording,magnetic recording media such as a magnetic tape and magnetic disk havebeen increasingly and strongly required to have a higher performance,namely, a higher recording density, higher output characteristic, inparticular, an improved frequency characteristic and a lower noiselevel.

Various attempts have been made at both enhancing the properties ofmagnetic particles and reducing the thickness of a magnetic recordinglayer in order to improve these properties of a magnetic recordingmedium.

The enhancement of the properties of magnetic particles will first bedescribed.

Magnetic particles are required to have, in order to satisfy theabove-described demands on a magnetic recording medium, properties suchas a high coercive force and a large saturation magnetization.

As magnetic particles suitable for high-output and high-densityrecording, acicular magnetic particles containing iron as a mainingredient which are obtained by heat-treating acicular goethiteparticles or acicular hematite particles in a reducing gas are widelyknown.

Acicular magnetic particles containing iron as a main ingredient have ahigh coercive force and a large saturation magnetization, since theacicular magnetic particles containing iron as a main ingredient usedfor a magnetic recording medium are very fine particles having aparticle size of not more than 1 μm, particularly, 0.01 to 0.3 μm.Therefore, such particles easily corrode, and the magnetic propertiesthereof are deteriorated, especially, the saturation magnetization andthe coercive force are reduced.

In order to maintain the characteristics of a magnetic recording mediumwhich uses magnetic particles containing iron as a main ingredient asthe magnetic particles, over a long period, it is strongly demanded tosuppress the corrosion of the acicular magnetic particles containingiron as a main ingredient as much as possible.

A reduction in the thickness of a magnetic recording layer will now bedescribed.

Video tapes have recently been required more and more to have a higherpicture quality, and the frequencies of carrier signals recorded inrecent video tapes are higher than those recorded in conventional videotapes. In other words, the signals in the short-wave region have come tobe used, and as a result, the magnetization depth from the surface of amagnetic tape has come to be remarkably small.

With respect to short wavelength signals, a reduction in the thicknessof a magnetic recording layer is also strongly demanded in order toimprove the high output characteristics, especially, an S/N ratio of amagnetic recording medium. This fact is described, for example, on page312 of Development of Magnetic Materials and Technique for HighDispersion of Magnetic Powder, published by Sogo Gijutsu Center Co.,Ltd. (1982), “ . . . the conditions for high-density recording in acoated-layer type tape are that the noise level is low with respect tosignals having a short wavelength and that the high outputcharacteristics are maintained. To satisfy these conditions, it isnecessary that the tape has large coercive force Hc and residualmagnetization Br, . . . and the coating film has a smaller thickness . .. ”.

Development of a reduction in the thickness of a magnetic recordinglayer has caused some problems.

Firstly, it is necessary to make a magnetic recording layer smooth andto eliminate the non-uniformity of thickness. As well known, in order toobtain a smooth magnetic recording layer having a uniform thickness, thesurface of the substrate must also be smooth. This fact is described onpages 180 and 181 of Materials for Synthetic Technology-Causes ofFriction and Abrasion of Magnetic Tape and Head Running System andMeasures for Solving the Problem (hereinunder referred to as “Materialsfor Synthetic Technology” (1987), published by the Publishing Departmentof Technology Information Center, “ . . . the surface roughness of ahardened magnetic layer depends on the surface roughness of thesubstrate (back surface roughness) so largely as to be approximatelyproportional, . . . , since the magnetic layer is formed on thesubstrate, the more smooth the surface of the substrate is, the moreuniform and larger head output is obtained, and the more the SIN ratiois improved.”

Secondly, there has been caused a problem in the strength of anon-magnetic substrate such as a base film with a tendency of thereduction in the thickness of the non-magnetic substrate in response tothe demand for a thinner magnetic layer. This fact is described, forexample, on page 77 of the above-described Development of MagneticMaterials and Technique for High Dispersion of Magnetic Powder, “ . . .Higher recording density is a large problem assigned to the presentmagnetic tape. This is important in order to shorten the length of thetape and to miniaturize the size of a cassette and to enable long-timerecording. For this purpose, it is necessary to reduce the thickness ofa substrate . . . With the tendency of reduction in the film thickness,the stiffness of the tape also reduces to such an extent as to makesmooth travel in a recorder difficult. Therefore, improvement of thestiffness of a video tape both in the machine direction and in thetransverse direction is now strongly demanded . . . ”

The end portion of a magnetic recording medium such as a magnetic tape,especially, a video tape is judged by detecting a portion of themagnetic recording medium at which the light transmittance is large by avideo deck. If the light transmittance of the whole part of a magneticrecording layer is made large by the thinner magnetic recording mediumor the ultrafine magnetic particles dispersed in the magnetic recordinglayer, it is difficult to detect the portion having a large lighttransmittance by a video deck. For reducing the light transmittance ofthe whole part of a magnetic recording layer, carbon black or the likeis added to the magnetic recording layer. It is, therefore, essential toadd carbon black or the like to a magnetic recording layer in thepresent video tapes.

However, addition of a large amount of non-magnetic particles such ascarbon black impairs not only the enhancement of the recording densitybut also the development of a thinner recording layer. Therefore inorder to reduce the magnetization depth from the surface of the magnetictape and to produce a thinner magnetic recording layer, it is stronglydemanded to reduce, as much as possible, the quantity of non-magneticparticles such as carbon black which are added to a magnetic recordinglayer.

It is also strongly demanded that the light transmittance of a magneticrecording layer should be small even if the carbon black or the likewhich is added to the magnetic recording layer is reduced to a smallamount. From this point of view, improvements in a substrate are now instrong demand.

Further, in order to reduce not only the above-mentioned opticaltransmittance but also surface resistivity of the magnetic recordingmedium, carbon black has been conventionally added to a magneticrecording layer thereof.

The use of carbon black in the magnetic recording medium is described inmore detail below.

In the case where the magnetic recording medium has a high surfaceresistivity, the electrostatically charged amount on the magneticrecording medium is increased, so that cutting wastes of magneticrecording media or dusts are attached to the surface of magneticrecording medium upon production or use of the magnetic recordingmedium, thereby increasing occurrence of drop-out.

Consequently, in order to lower the surface resistivity of the magneticrecording medium to about 10⁸ Ωcm, a conductive compound such as carbonblack has been generally added to a magnetic recording layer thereof inan amount of not less than about 5 parts by weight based on 100 parts byweight of magnetic particles used therein.

However, such an increase in amount of non-magnetic substance such ascarbon black in the magnetic recording layer tends to cause thedeterioration in signal recording property and inhibit the reduction inthickness of the magnetic recording layer.

Various efforts have been made to improve a base film for a magneticrecording layer with a demand for a thinner magnetic recording layer anda thinner non-magnetic substrate. A magnetic recording medium having atleast one undercoat layer (hereinunder referred to “non-magneticundercoat layer”) comprising a binder resin and non-magnetic particlescontaining iron as a main ingredient such as hematite particles whichare dispersed therein, on a non-magnetic substrate such as a base filmhas been proposed and put to practical use (Japanese Patent Publication(KOKOKU) No. 6-93297 (1994), Japanese Patent Application Laid-Open(KOKAI) Nos. 62-159338 (1987), 63-187418 (1988), 4-167225 (1992),4-325915 (1992), 5-73882 (1993), 5-182177 (1993), 5-347017 (1993),6-60362 (1994), 9-35245 (1997), etc.)

Further, various attempts for reducing the content of carbon black inthe magnetic recording layer and lowering the surface resistivity of themagnetic recording medium as low as possible, have been conducted. Forexample, it is known that the surfaces of non-magnetic particlesdispersed in the above-mentioned non-magnetic undercoat layer are coatedwith a tin compound or an antimony compound (Japanese Patent Nos.2566088 and 2566089, Japanese Patent Publication (KOKOKU) No.5-33446(1993), Japanese Patent Applications Laid-open (KOKAI) Nos.6-60360(1994), 7-176030(1995), 8-50718(1996), 8-203063(1996), 8-255334,9-27116(1997) or the like).

For example, Japanese Patent Application Laid-Open (KOKAI) No. 5-182177(1993) discloses a magnetic recording medium comprising: a non-magneticsubstrate; a non-magnetic undercoat layer formed on the non-magneticsubstrate and produced by dispersing inorganic particles in a binderresin; and a magnetic layer formed on the non-magnetic undercoat layerand produced by dispersing ferromagnetic particles in a binder resinwhile the non-magnetic undercoat layer is wet; wherein the magneticlayer has a thickness of not more than 1.0 μm in a dried state, thenon-magnetic undercoat layer contains non-magnetic inorganic particleswith surface layers coated with an inorganic oxide, the inorganic oxidecoating the surfaces of the non-magnetic inorganic particles containedin the non-magnetic undercoat layer is at least one selected from thegroup consisting of Al₂O₃, SiO₂ and ZrO₂, and the amount of theinorganic oxide coating the non-magnetic inorganic particles is 1 to 21wt % in the case of Al₂O₃, 0.04 to 20 wt % in the case of SiO₂, and 0.05to 15 wt % in the case of ZrO₂, base on the total weigh of the magneticinorganic particles.

In Japanese Patent No. 2566088, there is described a magnetic recordingmedium comprising a non-magnetic substrate, a non-magnetic undercoatlayer formed on the non-magnetic substrate, comprising a binder resinand non-magnetic inorganic particles dispersed in the binder resin andcoated with at least one oxide selected from the group consisting ofAl₂O₃, SiO₂, ZrO₂, Sb₂O₃ and ZnO, and a magnetic uppercoat layer formedon the non-magnetic undercoat layer, comprising a binder resin andferromagnetic particles dispersed in the binder resin, wherein themagnetic uppercoat layer has a dry thickness of not more than 1.0 μm;the non-magnetic undercoat layer has a dry thickness of 0.5 to 10 μm;and the ferromagnetic particles have a major axial diameter of not morethan 0.3 μm.

At present, there has been more demanded non-magnetic particles fornon-magnetic undercoat layer of a magnetic recording medium, which arecapable of furnishing a non-magnetic undercoat layer having excellentsurface smoothness and mechanical strength by dispersing thenon-magnetic particles in a binder resin; which are capable offurnishing a magnetic recording layer having a surface smoothness and athin and uniform thickness when the magnetic recording layer is formedon the non-magnetic undercoat layer; which are capable of furnishing amagnetic recording medium having a low transmittance and a low surfaceresistivity; and which are capable of preventing the corrosion ofmagnetic particles containing iron as a main ingredient, which aredispersed in the magnetic recording layer. However, such non-magneticparticles have not been furnished yet.

That is, it has been reported that the above-mentioned conventionalmagnetic recording medium using hematite particles as non-magneticparticles for non-magnetic undercoat layer thereof, are improved insurface smoothness and mechanical strength of the non-magnetic undercoatlayer; is capable of forming a magnetic recording layer having a surfacesmoothness, and a thin and uniform thickness upon the formation of themagnetic recording layer; and exhibit a low transmittance. However,these properties reported are still unsatisfactory. Especially, asdescribed in Comparative Examples hereinafter, the surface resistivityof these conventional magnetic recording medium is as high as 10⁹ to10¹¹ Ωcm.

On the other hand, in the case of the magnetic recording medium havingthe non-magnetic undercoat layer containing non-magnetic particlescoated with a tin compound or an antimony compound and dispersed in abinder resin, the non-magnetic undercoat layer is deteriorated insurface smoothness and mechanical strength, though the surfaceresistivity thereof is low. Accordingly, the magnetic recording layerformed on such a non-magnetic undercoat layer necessarily has a roughsurface and an uneven thickness, and exhibit an unsatisfactorytransmittance.

Further, there has also been pointed out such a problem that themagnetic particles containing iron as a main ingredient, which aredispersed in the magnetic recording layer, undergo server corrosionafter the production of the magnetic recording medium, thereby causingthe considerable deterioration in magnetic properties thereof.

As a result of the present inventors' earnest studies for solving theabove-mentioned problems, it has been found that by coating at least apart of surfaces of specific acicular hematite particles with an oxideof tin or an oxide of tin and antimony, and controlling the pH value tonot less than 8 and contents of soluble sodium salts and solublesulfates to a certain range, the obtained high-density acicular hematiteparticles exhibit a low surface resistivity and an excellentdispersibility in a vehicle. The present invention has been attained onthe basis of this finding.

SUMMARY OF THE INVENTION:

It is an object of the present invention to provide non-magneticparticles for non-magnetic undercoat layer of a magnetic recordingmedium, which are capable of furnishing a non-magnetic undercoat layerhaving excellent surface smoothness and mechanical strength bydispersing the non-magnetic particles in a binder resin; which arecapable of furnishing a magnetic recording layer having a surfacesmoothness and a thin and uniform thickness upon the formation of themagnetic recording layer; which are capable of furnishing a magneticrecording medium having a low transmittance and a low surfaceresistivity; and which are capable of preventing the corrosion of metalmagnetic particles containing iron as a main ingredient, which aredispersed in the magnetic recording layer.

It is another object of the present invention to provide a non-magneticundercoat layer which has excellent surface smoothness and mechanicalstrength, which is capable of forming thereon a magnetic recordinglayer, which is capable of imparting an excellent surface smoothness, alow transmittance and a low surface resistivity to the magneticrecording layer when formed on the non-magnetic undercoat layer, andwhich is capable of preventing metal magnetic particles containing ironas a main ingredient, which are dispersed in the magnetic recordinglayer, from being corroded, thereby inhibiting the deterioration inmagnetic properties thereof.

It is other object of the present invention to provide a magneticrecording medium which has an excellent surface smoothness, a lowtransmittance and a low surface resistivity, and in which the corrosionof metal magnetic particles containing iron as a main ingredient, whichare dispersed in the magnetic recording layer, is prevented, therebyinhibiting the deterioration in magnetic properties thereof.

To accomplish the aim, in a first aspect of the present invention, thereis provided high-density acicular hematite particles comprising acicularhematite particles and a coat comprising an oxide of tin or oxides oftin and antimony, formed on at least a part of surfaces of the acicularhematite particles; and having an average major axial diameter of notmore than 0.3 μm, a pH value of not less than 8, a soluble sodium saltcontent of not more than 300 ppm (calculated as Na) and a solublesulfate content of not more than 150 ppm (calculated as SO₄).

In a second aspect of the present invention, there is providedhigh-density acicular hematite particles comprising acicular hematiteparticles, a first coat comprising an oxide of tin or oxides of tin andantimony, formed on at least a part of surfaces of the acicular hematiteparticles, and a second coat comprising at least one compound selectedfrom the group consisting of a hydroxide of aluminum, an oxide ofaluminum, a hydroxide of silicon and an oxide of silicon, formed on atleast a part of surfaces of said high-density acicular hematiteparticles; and

having an average major axial diameter of not more than 0.3 μm, a pHvalue of not less than 8, a soluble sodium salt content of not more than300 ppm (calculated as Na) and a soluble sulfate content of not morethan 150 ppm (calculated as SO₄).

In a third aspect of the present invention, there is provided anon-magnetic undercoat layer comprising the high-density acicularhematite particles set forth in the first or second aspect and a binderresin, formed on a non-magnetic substrate.

In a fourth aspect of the present invention, there is provided amagnetic recording medium comprising:

a non-magnetic substrate;

a non-magnetic undercoat layer comprising the high-density acicularhematite particles set forth in the first or second aspect and a binderresin, formed on said non-magnetic substrate; and

a magnetic recording layer comprising magnetic particles containing ironas a main ingredient and a binder resin, formed on said non-magneticundercoat layer.

In a fifth aspect of the present invention, there is provided a processfor producing high-density acicular hematite particles set forth inclaim 1, comprising:

heat-dehydrating acicular goethite particles coated with a hydroxide oftin to obtain low-density acicular hematite particles;

heat-treating said low-density acicular hematite particles at atemperature of not less than 550° C. to obtain high-density acicularhematite particles coated with an oxide of tin;

wet-pulverizing a slurry containing said high-density acicular hematiteparticles;

adjusting the pH value of said slurry to not less than 13;

heat-treating said slurry at a temperature of not less than 80° C.; and

filtering said slurry to separate high-density acicular hematiteparticles therefrom, followed by washing with water and drying.

In a sixth aspect of the present invention, there is provided a processfor producing high-density acicular hematite particles set forth inclaim 1, comprising:

wet-pulverizing a slurry containing high-density acicular hematiteparticles obtained by heat-treating at a temperature of not less than550° C. low-density acicular hematite particles produced byheat-dehydrating acicular goethite particles coated with a sinteringpreventive agent,;

adjusting the pH value of said slurry to not less than 13;

heat-treating said slurry at a temperature of not less than 80° C.; and

filtering said slurry to separate high-density acicular hematiteparticles therefrom, followed by washing with water and drying;

treating the obtained high-density acicular hematite particles with anaqueous solution containing a tin compound to obtain high-densityacicular hematite particles coated with a hydroxide of tin; and

heat-treating said high-density acicular hematite particles coated witha hydroxide of tin at a temperature of not less than 300° C.

In a seventh aspect of the present invention, there is provided aprocess for producing high-density acicular hematite particles set forthin claim 1, comprising:

heat-dehydrating acicular goethite particles coated with hydroxides oftin and antimony to obtain low-density acicular hematite particles;

heat-treating said low-density acicular hematite particles at atemperature of not less than 550° C. to obtain high-density acicularhematite particles coated with oxides of tin and antimony;

wet-pulverizing a slurry containing said high-density acicular hematiteparticles;

adjusting the pH value of said slurry to not less than 13;

heat-treating said slurry at a temperature of not less than 80° C.; and

filtering said slurry to separate high-density acicular hematiteparticles therefrom, followed by washing with water and drying.

In an eighth aspect of the present invention, there is provided aprocess for producing high-density acicular hematite particles set forthin claim 1, comprising:

wet-pulverizing a slurry containing high-density acicular hematiteparticles obtained by heat-treating at a temperature of not less than550° C. low-density acicular hematite particles produced byheat-dehydrating acicular goethite particles coated with a sinteringpreventive agent,;

adjusting the pH value of said slurry to not less than 13;

heat-treating said slurry at a temperature of not less than 80° C.; and

filtering said slurry to separate high-density acicular hematiteparticles therefrom, followed by washing with water and drying;

treating the obtained high-density acicular hematite particles with anaqueous solution containing a tin compound and an antimony compound toobtain high-density acicular hematite particles coated with hydroxidesof tin and antimony; and

heat-treating said high-density acicular hematite particles coated withhydroxides of tin and antimony at a temperature of not less than 300° C.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described in detail below.

First, the high-density acicular hematite particles in which at least apart of the surface thereof is coated with an oxide of tin or an oxideof tin and antimony, are described.

The amount of the oxide of tin coated on surface of the particles isusually 0.5 to 500% by weight (calculated as Sn) based on the weight ofthe acicular hematite particles. When the amount of the oxide of tin isless than 0.5% by weight, the surface of the particles cannot besatisfactorily coated with the oxide of tin as a conductive substance,so that it becomes impossible to attain a sufficient effect of reducinga surface resistivity of the magnetic recording medium. On the otherhand, when the amount of the oxide of tin is more than 500% by weight,although a sufficient effect of reducing a surface resistivity of themagnetic recording medium can be obtained, the effect is alreadysaturated and, therefore, the use of such an excessive amount of theoxide of tin is meaningless. In view of the surface resistivity of theobtained magnetic recording medium and economy of the productionthereof, the amount of the oxide of tin is preferably 1.0 to 250% byweight, more preferably 2.0 to 200% by weight (calculated as Sn) basedon the weight of the acicular hematite particles.

The amount of the oxide of antimony coated on surfaces of the particlesis usually not more than 50% by weight, preferably 0.05 to 50% by weight(calculated as Sb) based on the weight of the acicular hematiteparticles. When the amount of the oxide of antimony is more than 50% byweight, although a sufficient effect of reducing a surface resistivityof the magnetic recording medium can be obtained, the effect is alreadysaturated and, therefore, the use of such an excessive amount of theoxide of antimony is meaningless. In view of the surface resistivity ofthe obtained magnetic recording medium and economy of the productionthereof, the amount of the oxide of antimony is more preferably 0.1 to25% by weight (calculated as Sb) based on the weight of the acicularhematite particles.

In the case where the surface of the particles are coated with the oxideof tin and antimony, the weight ratio of tin to antimony is usually 20:1to 1:1, preferably 15:1 to 2:1. When the amount of tin is less than thatof antimony, it may become difficult to effectively reduce a surfaceresistivity of the magnetic recording medium. When the weight ratio oftin to antimony exceeds 20, it may become difficult to more effectivelyreduce a surface resistivity of the magnetic recording medium, becausethe amount of tin is too small.

The high-density acicular hematite particles coated with the oxide oftin or the oxides of tin and antimony according to the present inventionhave an average major axial diameter of not more than 0.3 μm, a pH valueof not less than 8, a soluble sodium salt content of not more than 300ppm (calculated as Na) and a soluble sulfate content of not more than150 ppm (calculated as SO₄).

The high-density acicular hematite particles in the present inventionhave an aspect ratio (average major axial diameter/average minor axialdiameter) (hereinunder referred to merely as “aspect ratio”) of not lessthan 2:1, preferably not less than 3:1. The upper limit of the aspectratio is usually 20:1, preferably 10:1 with the consideration of thedispersibility in the vehicle. The shape of the acicular particles heremay have not only acicular but also spindle-shaped, rice ball-shaped orthe like.

When the aspect ratio is less than 2:1, it is difficult to obtain adesired film strength of the magnetic recording medium.

The average major axial diameter of the high-density acicular hematiteparticles of the present invention is not more than 0.3 μm, preferably0.005 to 0.3 μm. When the average major axial diameter exceeds 0.3 μm,the particle size is so large as to impair the surface smoothness. Withthe consideration of the dispersibility in the vehicle and the surfacesmoothness of the coated film, the more preferable average major axialdiameter is 0.02 to 0.2 μm.

The average minor axial diameter of the high-density acicular hematiteparticles of the present invention is usually 0.0025 to 0.15 μm. Whenthe average minor axial diameter is less than 0.0025 μm, dispersion inthe vehicle may be unfavorably difficult. On the other hand, when theaverage minor axial diameter exceeds 0.15 μm, the particle size may beapt to become so large as to impair the surface smoothness. With theconsideration of the dispersibility in the vehicle and the surfacesmoothness of the coated film, the more preferable average minor axialdiameter is 0.01 to 0.10 μm.

The BET specific surface area of the high-density acicular hematiteparticle of the present invention is usually not less than 35 m²/g. Whenit is less than 35 m²/g, the acicular hematite particles may be coarseor sintering may be sometimes caused between particles, which are apt toexert a deleterious influence on the surface smoothness of the coatedfilm. The BET surface area thereof is more preferably not less than 40m²/g, even more preferably not less than 45 m²/g, and the upper limitthereof is usually 150 m²/g. The upper limit is preferably 100 m²/g,more preferably 80 m²/g with the consideration of the dispersibility inthe vehicle.

The degree of densification (S_(BET)/S_(TEM)) of hematite particles isrepresented by the ratio of the specific surface area (S_(BET)) measuredby a BET method and the surface area (S_(TEM)) calculated from the majoraxial diameter and the minor axial diameter which were measured from theparticles in an electron micrograph.

The S_(BET)/S_(TEM) value of hematite particles according to the presentinvention is usually 0.5 to 2.5. When the S_(BET)/S_(TEM) value is lessthan 0.5, although the hematite particles have been densified, theparticles may adhere to each other due to sintering therebetween, andthe particle size may increase, so that a sufficient surface smoothnessof the coated film may be not obtained. On the other hand, when theS_(BET)/S_(TEM) value exceeds 2.5, there may be many pores in thesurfaces of particles and the dispersibility in the vehicle may becomeinsufficient. In consideration of the surface smoothness of the coatedfilm and the dispersibility in the vehicle, the S_(BET)/S_(TEM) value ispreferably 0.7 to 2.0, more preferably 0.8 to 1.6.

The major axial diameter distribution of the high-density acicularhematite particles of the present invention is preferably not more than1.50 in geometrical standard deviation. When it exceeds 1.50, the coarseparticles existent sometimes exert a deleterious influence on thesurface smoothness of the coated film. The major axial diameterdistribution is more preferably not more than 1.40, even more preferablynot more than 1.35 in geometrical standard deviation with theconsideration of the surface smoothness of the coated film. From thepoint of view of industrial productivity, the major axial diameterdistribution of the high-density acicular hematite particles obtained isusually 1.01 in geometrical standard deviation.

The pH value of the high-density acicular hematite particles of thepresent invention is not less than 8. When it is less than 8, themagnetic particles containing iron as a main ingredient contained in themagnetic recording layer formed on the non-magnetic undercoat layer aregradually corroded, thereby causing a deterioration in the magneticproperties. With the consideration of a corrosion preventive effect onthe magnetic particles containing iron as a main ingredient, the pHvalue of the particles is preferably not less than 8.5, more preferablynot less than 9.0. The upper limit is usually 12, preferably 11, morepreferably 10.5.

The content of soluble sodium salts in the high-density acicularhematite particles of the present invention is not more than 300 ppmsoluble sodium (calculated as Na). When it exceeds 300 ppm, the magneticparticles containing iron as a main ingredient contained in the magneticrecording layer formed on the non-magnetic undercoat layer are graduallycorroded, thereby causing a deterioration in the magnetic properties. Inaddition, the dispersion property of the high-density acicular hematiteparticles in the vehicle is easily impaired, and the preservation of themagnetic recording medium is deteriorated and efflorescence is sometimescaused in a highly humid environment. With the consideration of acorrosion preventive effect on the magnetic particles containing iron asa main ingredient, the content of soluble sodium salt is preferably notmore than 250 ppm, more preferably not more than 200 ppm, even morepreferably not more than 150 ppm. From the point of view of industrysuch as productivity, the lower limit thereof is preferably about 0.01ppm.

The content of soluble sulfate in the high-density acicular hematiteparticles of the present invention is not more than 150 ppm solublesulfate (calculated as SO₄). When it exceeds 150 ppm, the magneticparticles containing iron as a main ingredient contained in the magneticrecording layer formed on the non-magnetic undercoat layer are graduallycorroded, thereby causing a deterioration in the magnetic properties. Inaddition, the dispersion property of the high-density acicular hematiteparticles in the vehicle is easily impaired, and the preservation of themagnetic recording medium is deteriorated and efflorescence is sometimescaused in a highly humid environment. With the consideration of acorrosion preventive effect on the magnetic particles containing iron asa main ingredient, the content of soluble sulfate is preferably not.more than 70 ppm, more preferably not more than 50 ppm. From the pointof view of industry such as productivity, the lower limit thereof ispreferably about 0.01 ppm.

The high-density acicular hematite particles according to the presentinvention in which at least a part of the surface thereof is coated withthe oxide of tin or the oxide of tin and antimony, have a volumeresistivity of 10³ to 5×10⁷ Ωcm. When the volume resistivity of acicularhematite particles is more than 10⁸ Ωcm, it may become difficult toobtain a magnetic recording medium having a sufficiently low surfaceresistivity.

At least a part of the surfaces of the high-density acicular hematiteparticles coated of the present invention may be coated with at leastone selected from the group consisting of a hydroxide of aluminum, anoxide of aluminum, a hydroxide of silicon and an oxide of silicon. Whenthe acicular hematite particles coated with the above-described coatingmaterial are dispersed in a vehicle, they have an affinity with thebinder resin and it is easy to obtain a desired dispersibility.

The amount of aluminum hydroxide, aluminum oxide, silicon hydroxide orsilicon oxide used as the coating material is usually not less than 50wt %, preferably 0.01 to 50 wt % (calculated as Al or SiO₂). When it isless than 0.01 wt %, the dispersibility improving effect may beinsufficient. When the amount exceeds 50 wt %, the coating effectbecomes saturated, so that it is meaningless to add a coating materialmore than necessary. From the point of view of dispersibility in thevehicle, the preferable amount of coating material is preferably 0.05 to20 wt % (calculated as Al or SiO₂).

Various properties of the high-density acicular hematite particlescoated with a coating material of the present invention, such as aspectratio, average major axial diameter, average minor axial diameter, pHvalue, the content of soluble sodium salt, content of soluble sulfate,BET specific surface area, major axial diameter distribution, degree ofdensification, and volume resistivity are approximately equivalent invalues to those of the high-density acicular hematite particles of thepresent invention the surfaces of which are not coated with a coatingmaterial.

A non-magnetic undercoat layer and a magnetic recording medium accordingto the present invention will now be explained.

The magnetic medium of according to the present invention comprises anon-magnetic substrate, a non-magnetic undercoat layer and a magneticrecording layer.

The non-magnetic undercoat layer of the present invention is produced byforming a coating film on the non-magnetic substrate and drying thecoating film. The non-magnetic coating film is formed by applying to thesurface of the non-magnetic substrate a non-magnetic coating compositionwhich contains the high-density acicular hematite particles, a binderresin and a solvent.

As the non-magnetic substrate, the following materials which are atpresent generally used for the production of a magnetic recording mediumare usable as a raw material: a synthetic resin such as polyethyleneterephthalate, polyethylene, polypropylene, polycarbonate, polyethylenenaphthalate, polyamide, polyamideimide and polyimide; foil and plate ofa metal such as aluminum and stainless steel; and various kinds ofpaper. The thickness of the non-magnetic substrate varies depending uponthe material, but it is usually about 1.0 to 300 μm, preferably 2.0 to200 μm. In the case of a magnetic disc, polyethylene terephthalate isordinarily used as the non-magnetic substrate. The thickness thereof isusually 50 to 300 μm, preferably 60 to 200 μm. In the case of a magnetictape, when polyethylene terephthalate is used as the non-magneticsubstrate, the thickness thereof is usually 3 to 100 μm, preferably 4 to20 μm. When polyethylene naphthalate is used, the thickness thereof isusually 3 to 50 μm, preferably 4 to 20 μm. When polyamide is used, thethickness thereof is usually 2 to 10 μm, preferably 3 to 7 μm.

The thickness of the non-magnetic undercoat layer obtained by coatingthe non-magnetic substrate with a coating composition and drying thecoating film, is usually 0.2 to 10.0 μm, preferably 0.5 to 5.0 μm. Whenthe thickness is less than 0.2 μm, not only it is impossible toameliorate the surface roughness of the non-magnetic substrate but alsothe strength is insufficient.

As the binder resin in the present invention, the following resins whichare at present generally used for the production of a magnetic recordingmedium are usable: vinyl chloride-vinyl acetate copolymer, urethaneresin, vinyl chloride-vinyl acetate-maleic acid copolymer, urethaneelastomer, butadiene-acrylonitrile copolymer, polyvinyl butyral,cellulose derivative such as nitrocellulose, polyester resin, syntheticrubber resin such as polybutadiene, epoxy resin, polyamide resin,polyisocyanate, electron radiation curing acryl urethane resin andmixtures thereof. Each of these resin binders may contain a functionalgroup such as —OH, —COOH, —SO₃M, —OP₂M₂ and —NH₂, wherein M representsH, Na or K. With the consideration of the dispersibility of theparticles, a binder resin containing a functional group —COOH or —SO₃Mis preferable.

The mixing ratio of the high-density acicular hematite particles withthe binder resin is usually 5 to 2000 parts by weight, preferably 100 to1000 parts by weight based on 100 parts by weight of the binder resin.

It is possible to add a lubricant, a polishing agent, an antistaticagent, etc. which are generally used for the production of a magneticrecording medium to the non-magnetic undercoat layer.

The gloss of the coated film of the non-magnetic undercoat layercontaining high-density acicular hematite particles according to thepresent invention is usually 180 to 280%, preferably 185 to 280%, morepreferably 187 to 280% and the surface roughness Ra thereof is usually2.0 to 13.0 nm, preferably 2.0 to 11.0 nm, more preferably 2.0 to 10.0nm. The Young's modulus (relative value to a commercially availablevideo tape: AV T-120 produced by Victor Company of Japan, Limited)thereof is usually 115 to 150, preferably 120 to 150, more preferably125 to 150.

The magnetic recording medium according to the present invention isproduced by forming the non-magnetic undercoat layer formed on thenon-magnetic substrate, forming a coating film on the non-magneticundercoat layer by applying a coating composition containing magneticparticles containing iron as a main ingredient, a binder resin and asolvent, and drying the coating film to obtain a magnetic recordinglayer.

The magnetic particles containing iron as a main ingredient used in thepresent invention comprises iron or iron and at least one selected fromthe group consisting of Co, Al, Ni, P, Si, Zn, Ti, Cu, B, Nd, La and Y.Further, the following magnetic particles containing iron as a mainingredient may be exemplified.

1) Magnetic particles containing iron as a main ingredient comprisesiron and usually 0.05 to 10 wt%, preferably 0.1 to 7 wt % of aluminum(calculated as Al) based on the weight of the magnetic particlescontaining iron as a main ingredient.

2) Magnetic particles containing iron as a main ingredient comprisesiron; usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % of aluminum(calculated as Al) based on the weight of the magnetic particlescontaining iron as a main ingredient; and usually 0.05 to 40 wt %,preferably 1.0 to 35 wt %, more preferably 3 to 30 wt % of cobalt(calculated as Co) based on the weight of the magnetic particlescontaining iron as a main ingredient.

3) Magnetic particles containing iron as a main ingredient comprisesiron; usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % of aluminum(calculated as Al) based on the weight of the magnetic particlescontaining iron as a main ingredient; and usually 0.05 to 10 wt %,preferably 0.1 to 7 wt % of at least one selected from the groupconsisting of Nd, La and Y (calculated as the corresponding element)based on the weight of the magnetic particles containing iron as a mainingredient.

4) Magnetic particles containing iron as a main ingredient comprisesiron; usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % of aluminum(calculated as Al) based on the weight of the magnetic particlescontaining iron as a main ingredient; usually 0.05 to 40 wt %,preferably 1.0 to 35 wt %, more preferably 3 to 30 wt % of cobalt(calculated as Co) based on the weight of the magnetic particlescontaining iron as a main ingredient;; and usually 0.05 to 10 wt %,preferably 0.1 to 7 wt % of at least one selected from the groupconsisting of Nd, La and Y (calculated as the corresponding element)based on the weight of the magnetic particles containing iron as a mainingredient.

5) Magnetic particles containing iron as a main ingredient comprisesiron; usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % of aluminum(calculated as Al) based on the weight of the magnetic particlescontaining iron as a main ingredient; and usually 0.05 to 10 wt %,preferably 0.1 to 7 wt % of at least one selected from the groupconsisting of Ni, P, Si, Zn, Ti, Cu and B (calculated as thecorresponding element) based on the weight of the magnetic particlescontaining iron as a main ingredient.

6) Magnetic particles containing iron as a main ingredient comprisesiron; usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % of aluminum(calculated as Al) based on the weight of the magnetic particlescontaining iron as a main ingredient; usually 0.05 to 40 wt %,preferably 1.0 to 35 wt %, more preferably 3 to 30 wt % of cobalt(calculated as Co) based on the weight of the magnetic particlescontaining iron as a main ingredient; and usually 0.05 to 10 wt %,preferably 0.1 to 7 wt % of at least one selected from the groupconsisting of Ni, P, Si, Zn, Ti, Cu and B (calculated as thecorresponding element) based on the weight of the magnetic particlescontaining iron as a main ingredient.

7) Magnetic particles containing iron as a main ingredient comprisesiron; usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % of aluminum(calculated as Al) based on the weight of the magnetic particlescontaining iron as a main ingredient; usually 0.05 to 10 wt %,preferably 0.1 to 7 wt % of at least one selected from the groupconsisting of Nd, La and Y (calculated as the corresponding element)based on the weight of the magnetic particles containing iron as a mainingredient; and usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % of atleast one selected from the group consisting of Ni, P, Si, Zn, Ti, Cuand B (calculated as the corresponding element) based on the weight ofthe magnetic particles containing iron as a main ingredient.

8) Magnetic particles containing iron as a main ingredient comprisesiron; usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % of aluminum(calculated as Al) based on the weight of the magnetic particlescontaining iron as a main ingredient; usually 0.05 to 40 wt %,preferably 1.0 to 35 wt %, more preferably 3 to 30 wt % of cobalt(calculated as Co) based on the weight of the magnetic particlescontaining iron as a main ingredient; usually 0.05 to 10 wt %,preferably 0.1 to 7 wt % of at least one selected from the groupconsisting of Nd, La and Y (calculated as the corresponding element)based on the weight of the magnetic particles containing iron as a mainingredient; and usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % of atleast one selected from the group consisting of Ni, P, Si, Zn, Ti, Cuand B (calculated as the corresponding element) based on the weight ofthe magnetic particles containing iron as a main ingredient.

The iron content in the particles is the balance, and is preferably 50to 99 wt %, more preferably 60 to 95 wt % (calculated as Fe) based onthe weight of the magnetic particles containing iron as a mainingredient.

The magnetic particles containing iron as a main ingredient comprising(i) iron and Al; (ii) iron, Al and Co, (iii) iron, Al and at least onerare-earth metal such as Nd, La and Y, or (iv) iron, Al, Co and at leastone rare-earth metal such as Nd, La and Y, are preferable from the pointof the durability of the magnetic recording medium. Further, themagnetic particles containing iron as a main ingredient comprising iron,Al and at least one rare-earth metal such as Nd, La and Y, are morepreferable.

The acicular magnetic particles containing iron as a main ingredientused in the present invention have an average major axial diameter ofusually 0.01 to 0.50 μm, preferably 0.03 to 0.30 μm, more preferably0.03 to 0.25 μm, an average minor axial diameter of usually 0.0007 to0.17 μm, preferably 0.003 to 0.10 μm, and an aspect ratio of usually notless than 3:1, preferably and not less than 5:1. The upper limit of theaspect ratio is usually 15:1, preferably 10:1 with the consideration ofthe dispersibility in the vehicle. The shape of the acicular magneticparticles containing iron as a main ingredient may have not onlyacicular but also. a spindle-shaped, rice ball-shaped or the like.

As to the magnetic properties of the acicular magnetic particlescontaining iron as a main ingredient used in the present invention, thecoercive force is preferably 1200 to 3200 Oe, more preferably 1500 to3200 Oe, and the saturation magnetization is preferably 100 to 170emu/g, more preferably 130 to 170 emu/g with the consideration of theproperties such as high-density recording.

As the binder resin for the magnetic recording layer, the same binderresin as that used for the production of the non-magnetic undercoatlayer is usable.

The thickness of the magnetic recording layer obtained by applying themagnetic coating composition to the non-magnetic undercoat layer anddried, is ordinarily in the range of 0.01 to 5.0 μm. When the thicknessis less than 0.01 μm, uniform coating may be difficult, so thatunfavorable phenomenon such as unevenness on the coating surface isobserved. On the other hand, when the thickness exceeds 5.0 μm, it maybe difficult to obtain desired signal recording property due to aninfluence of diamagnetism. The preferable thickness is in the range of0.05 to 1.0 μm.

The mixing ratio of the acicular magnetic particles containing iron as amain ingredient with the binder resin in the magnetic recording layer isusually 200 to 2000 parts by weight, preferably 300 to 1500 parts byweight based on 100 parts by weight of the binder resin.

It is possible to add a lubricant, a polishing agent, an antistaticagent, etc. which are generally used for the production of a magneticrecording medium to the magnetic recording layer.

The magnetic recording medium according to the present invention has acoercive force of usually 900 to 3500 Oe, preferably 1000 to 3500 Oe,more preferably 1500 to 3500 Oe; a squareness (residual magnetic fluxdensity Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95,preferably 0.87 to 0.95; a gloss (of the coating film) of usually 195 to300%, preferably 200 to 300%; a surface roughness Ra (of the coatingfilm) of usually not more than 11.0 nm, preferably 1.0 to 10.0 nm, morepreferably 1.0 to 9.0 nm; a Young's modulus (relative value to acommercially available video tape: AV T-120 produced by Victor Companyof Japan, Limited) of usually not less than 125, preferably not lessthan 130; a linear adsorption coefficient (of the coating film) ofusually 1.10 to 2.00 μm⁻¹, preferably 1.20 to 2.00 μm⁻¹. ; and a surfaceresistivity (of the coating film) of usually 10⁴ to 5×10⁸ Ω/sq,preferably 10⁴ to 4.5×10⁸ Ω/sq, more preferably 104 to 4×10⁸ Ω/sq.

The corrosiveness represented by a percentage (%) of change in thecoercive force is usually not more than 10.0%, preferably not more than9.5%, and the corrosiveness represented by a percentage (%) of change inthe saturation magnetic flux density Bm is usually not more than 10.0%,preferably not more than 9.5%.

Next, the process for producing the high-density acicular hematiteparticles coated with an oxide of tin or both an oxide of tin and anoxide of antimony according to the present invention, is describedbelow.

As a starting material for the acicular hematite particles, there may beused acicular goethite particles.

In order to produce the high-density acicular hematite particles of thepresent invention, acicular goethite particles are produced. Aciculargoethite particles are produced by an ordinary method:

(A) a method of oxidizing a suspension having a pH value of not lessthan 11 and containing colloidal ferrous hydroxide particles which isobtained by adding not less than an equivalent of an alkali hydroxidesolution to an aqueous ferrous salt solution, by passing anoxygen-containing gas thereinto at a temperature of not higher than 80°C.;

(B) a method of producing acicular goethite particles by oxidizing asuspension containing FeCO₃ which is obtained by reacting an aqueousferrous salt solution with an aqueous alkali carbonate solution, bypassing an oxygen-containing gas thereinto after aging , if necessary,the suspension;

(C) a method of growing acicular seed goethite particles by oxidizing aferrous hydroxide solution containing colloidal ferrous hydroxideparticles which is obtained by adding less than an equivalent of analkali hydroxide solution or an alkali carbonate solution to an aqueousferrous salt solution, by passing an oxygen-containing gas thereinto,thereby producing acicular seed goethite particles, adding not less thanan equivalent of an alkali hydroxide solution to the Fe²⁺ in the aqueousferrous salt solution, to the aqueous ferrous salt solution containingthe acicular goethite seed particles, and passing an oxygen-containinggas into the aqueous ferrous salt solution; and

(D) a method of growing acicular seed goethite particles by oxidizing aferrous hydroxide solution containing colloidal ferrous hydroxideparticles which is obtained by adding less than an equivalent of analkali hydroxide solution or an alkali carbonate solution to an aqueousferrous salt solution, by passing an oxygen-containing gas thereinto,thereby producing acicular seed goethite particles, and growing theobtained acicular seed goethite particles in an acidic or neutralregion.

Elements other than Fe such as Ni, Zn, P, Al and Si, which are generallyadded in order to enhance various properties of the particles such asthe major axial diameter, the minor axial diameter and the aspect ratio,may be added during the reaction system for producing the goethiteparticles.

The acicular goethite particles obtained have an average major axialdiameter of usually 0.005 to 0.4 μm, an average minor axial diameter ofusually 0.0025 to 0.20 μm and a BET specific of about usually 50 to 250m²/g, and contain ordinarily soluble sodium salts of 300 to 1500 ppmsoluble sodium (calculated as Na) and ordinarily soluble sulfates of 100to 3000 ppm soluble sulfate (calculated as SO₄).

The surfaces of the above-mentioned acicular goethite particles are thencoated with a hydroxide of tin or hydroxides of tin and antimony.

In the coating-treatment, a tin compound or a tin compound and anantimony compound is added to a water suspension obtained by dispersingthe acicular goethite particles in an aqueous solution. The suspensionis stirred and if required, the pH value of the suspension is adjustedproperly to coat the acicular goethite particles with the hydroxide oftin or the hydroxides of tin and antimony. Then, the suspension is thenfiltered to separate the coated acicular goethite particles therefrom.The coated acicular goethite particles is further washed with water,dried and pulverized.

As the tin compound added, there may be exemplified alkali stannatessuch as sodium stannate, tin salts such as stannous chloride, stannicchloride, stannous sulfate, stannic sulfate, stannous nitrate, stannicnitrate, stannous acetate or stannic acetate, or the like. The amount ofthe tin compound added is usually 0.5 to 500% by weight, preferably 1 to250% by weight (calculated as Sn) based on the weight of the aciculargoethite particles. When the amount of the tin compound added is lessthan 0.5% by weight, the acicular goethite particles cannot besufficiently coated with the hydroxide of tin. On the other hand, whenthe amount of the tin compound added is more than 500% by weight, theeffect by the addition is saturated and, therefore, the addition of suchan excessive amount of the tin compound is meaningless.

As the antimony compound added, there may be exemplified antimony saltssuch as antimonous chloride, antimonic chloride or antimony sulfate. Theamount of the antimony compound added is usually not more than 50% byweight, preferably 0.05 to 50% by weight (calculated as Sb) based on theweight of the acicular goethite particles. When the amount of theantimony compound added is more than 50% by weight, the effect by theaddition is saturated and, therefore, the addition of such an excessiveamount of the antimony compound is meaningless.

The thus obtained acicular goethite particles coated with the hydroxideof tin or the hydroxides of tin and antimony are heated at a temperatureas high as not less than 550° C. to produce high-density acicularhematite particles. Alternatively, the coated acicular goethiteparticles may be heat-dehydrated at a temperature of 250 to 500° C. formlow-density acicular hematite particles, and then, are heat-treated at atemperature as high as not less than 550° C. to produce high-densityacicular hematite particles. In order to obtain the high-densityacicular hematite particles maintaining the shape or configuration oforiginal acicular goethite particles, the latter method is preferred.

It is preferred to coat the particles with a sintering preventive beforethe heat-treatment at a high temperature in order to obtain high-densityacicular hematite particles which retain the shapes of the aciculargoethite particles. The acicular goethite particles coated with asintering preventive contain soluble sodium salts of usually 500 to 2000ppm soluble sodium (calculated as Na) and soluble sulfates of usually300 to 3000 ppm soluble sulfate (calculated as SO₄), and have the BETspecific surface area of usually about 50 to 250 m²/g. Thecoating-treatment using a sintering preventive is composed of the stepsof: adding a sintering preventive to an aqueous suspension containingthe acicular goethite particles, mixing and stirring the suspension,filtering out the particles, washing the particles with water, anddrying the particles.

Incidentally, in the case of the acicular goethite particles coated withthe hydroxide of tin or the hydroxides of tin and antimony, thehydroxide of tin or the hydroxides of tin and antimony works on assintering-preventive agent, and therefore, such coated acicular goethiteparticles may further be coated with the sintering-preventive agent.

The amount of sintering preventive existent on the surfaces of theacicular hematite particles of the present invention varies dependingupon various conditions such as the kind of sintering preventive, the pHvalue thereof in an aqueous alkali solution and the heating temperature,it is usually not more than 10 wt %, preferably 0.05 to 10 wt % based onthe total weight of the particles.

As the sintering preventive, sintering preventives generally used areusable. For example, phosphorus compounds such as sodiumhexametaphosphate, polyphospholic acid and orthophosphoric acid; siliconcompounds such as #3 water glass, sodium orthosilicate, sodiummetasilicate and colloidal silica; boron compounds such as boric acid;aluminum compounds including aluminum salts such as aluminum acetate,aluminum sulfate, aluminum chloride and aluminum nitride, alkalialuminate such as sodium aluminate, and alumina sol and aluminumhydroxide; and titanium compounds such as titanyl sulfate may beexemplified.

The low-density acicular hematite particles obtained by heat-treatingthe acicular goethite particles coated with a sintering preventive at atemperature of 250 to 500° C. have an average major axial diameter ofusually 0.005 to 0.30 μm, an average minor axial diameter of usually0.0025 to 0.15 μm, a BET specific surface area of usually about 70 to350 m²/g and contain soluble sodium salts of usually 500 to 2000 ppmsoluble sodium (calculated as Na) and soluble sulfates of usually 300 to4000 ppm soluble sulfate (calculated as SO₄). When the temperature forheat-treating the goethite particles is less than 250° C., thedehydration reaction takes a long time. On the other hand, When thetemperature exceeds 500° C. , the dehydration reaction is abruptlybrought out, so that it is difficult to retain the shapes because thesintering between particles is caused. The low-density acicular hematiteparticles obtained by heat-treating the goethite particles at a lowtemperature are low-density particles having a large number ofdehydration pores through which H₂O is removed from the goethiteparticles and the BET specific surface area thereof is about 1.2 to 2times larger than that of the acicular goethite particles as thestarting material.

The low-density hematite particles are then heat-treated at atemperature of not less than 550° C. to obtain a high-density acicularhematite particles. The upper limit of the heating temperature ispreferably 850° C. The high-density hematite particles contain solublesodium salts of usually 500 to 4000 ppm soluble sodium (calculated asNa) and soluble sulfates of usually 300 to 5000 ppm soluble sulfate(calculated as SO₄), and the BET specific surface area thereof isusually about 35 to 150 m²/g.

When the heat-treating temperature is less than 550° C., since thedensification is insufficient, a large number of dehydration pores existwithin and on the surface of the hematite particles, so that thedispersion in the vehicle is insufficient. Further, when thenon-magnetic undercoat layer is formed from these particles, it isdifficult to obtain a coated film having a smooth surface. On the otherhand, when the temperature exceeds 850° C., although the densificationof the hematite particles is sufficient, since sintering is caused onand between particles, the particle size increases, so that it isdifficult to obtain a coated film having a smooth surface.

The obtained acicular hematite particles are pulverized by adry-process, and formed into a slurry. The obtained slurry is thenpulverized by a wet-process so as to deagglomerate coarse particles. Inthe wet-pulverization, ball mill, sand grinder, colloid mill or the likeis used and wet-pulverization is conducted until coarse particles havinga particle size of at least 44 μm are substantially removed. That is,the wet-pulverization is carried out until the amount of the coarseparticles having a particle size of not less than 44 μm becomes tousually not more than 10% by weight, preferably not more than 5% byweight, more preferably 0% by weight based on the total weight of theparticles. When the amount of the coarse particles having a particlesize of not less than 44 μm is more than 10% by weight, the effect oftreating the particles in an aqueous alkali solution at the next step isnot attained.

The acicular hematite particles with coarse particles removed therefromare heat-treated in a slurry at a temperature of usually not less than80° C. after the pH value of the slurry is adjusted to not less than 13by adding an aqueous alkali solution such as sodium hydroxide.

The concentration of the alkali suspension containing the acicularhematite particles and having a pH value of not less than 13 ispreferably 50 to 250 g/liter.

When the pH value of the alkali suspension containing the acicularhematite particles is less than 13, it is impossible to effectivelyremove the solid crosslinking caused by the sintering preventive whichexists on the surfaces of the hematite particles, so that it isimpossible to wash out the soluble sodium slat, soluble sulfate, etc.existing within and on the surfaces of the particles. The upper limit ofthe pH value is usually about 14. When the effect of removing the solidcrosslinking caused by the sintering preventive which exists on thesurfaces of the hematite particles, the effect of washing out thesoluble sodium slat, soluble sulfate, etc., and the effect of removingthe alkali which adheres to the surfaces of hematite particles in theprocess of the heat-treatment of the aqueous alkali suspension are takeninto consideration, the preferable pH value thereof is in the range of13.1 to 13.8.

The heat-treating temperature in the aqueous alkali suspension whichcontains the acicular hematite particles and has a pH value of not lessthan 13, is usually not less than 80° C., preferably not less than 90°C. If the temperature is less than 80° C., it is difficult toeffectively remove the solid crosslinking caused by the sinteringpreventive which exists on the surfaces of the hematite particles. Theupper limit of the heating temperature is preferably 103° C., morepreferably 100° C. When the heating temperature exceeds 103° C.,although it is possible to effectively remove the solid crosslinking,since an autoclave or the like is necessary or solution boils under anormal pressure, it is not advantageous from the point of view ofindustry.

The acicular hematite particles heat-treated in the aqueous alkalisuspension are, thereafter, filtered out and washed with water by anordinary method so as to remove the soluble sodium salt and solublesulfate which are washed out of the interiors and the surfaces of theparticles and the alkali such as sodium or the like adhered to thesurfaces of the hematite particles in the process of heat-treatment withthe aqueous alkali suspension, and then dried.

As the method of washing the particles with water, a method generallyindustrially used such as a decantation method, a dilution method usinga filter thickener and a method of passing water into a filter press isadopted.

If the soluble sodium salt and soluble sulfate which are containedwithin the high-density hematite particles are washed out with water,even if soluble sodium salt and soluble sulfate adhere to the surfaceswhen the surfaces of the hematite particles are coated with a coatingmaterial in a subsequent step, for example, the later-described coatingstep, they can be easily removed by water-washing.

Alternatively, the high-density acicular hematite particles coated withthe oxide of tin or the oxides of tin and antimony may be produced bythe following method. That is, by using acicular goethite particlesuncoated with the hydroxide of tin or the hydroxides of tin and antimonybut coated with the sintering-preventive agent solely as a startingmaterial, high-density acicular hematite particles uncoated with thehydroxide of tin or the hydroxides of tin and antimony are firstproduced. The obtained high-density acicular hematite particles areheated in an aqueous alkaline solution, and then filtered and washedwith water by ordinary methods. Next, the thus treated high-densityacicular hematite particles are coated with the hydroxide of tin or thehydroxides of tin and antimony in the same manner as the above-mentionedcoating-treatment of the acicular goethite particles. Thereafter, thehigh-density acicular hematite particles coated with the hydroxide oftin or the hydroxides of tin and antimony are heated at a temperature ofusually not less than 300° C., preferably 350 to 850° C., to convert thehydroxide of tin or the hydroxides of tin and antimony on surfaces ofthe high-density acicular hematite particles, into the oxide of tin orthe oxides of tin and antimony, thereby obtaining the high-densityacicular hematite particles coated with the oxide of tin or the oxidesof tin and antimony.

The high-density acicular hematite particles coated with the oxide oftin or the oxides of tin and antimony according to the presentinvention, may be further coated with at least one compound selectedfrom the group consisting of a hydroxide of aluminum, an oxide ofaluminum, a hydroxide of silicon and an oxide of silicon, if required.

In the coating-treatment, an aluminum compound, a silicon compound orboth the aluminum and silicon compounds are added to a water suspensionobtained by dispersing the high-density acicular hematite particlescoated with the oxide of tin or the oxides of tin and antimony in anaqueous solution. The suspension is stirred and if required, the pHvalue of the suspension is adjusted properly to coat at least a part ofthe surface of the high-density acicular hematite particles with thehydroxide of aluminum, the oxide of aluminum, the hydroxide of siliconor the oxide of silicon. The suspension is then filtered to separate thecoated high-density acicular hematite particles therefrom. The coatedhigh-density acicular hematite particles is further washed with water,dried and pulverized. If required, the high-density acicular hematiteparticles may be subjected to deaeration, compaction or othertreatments.

As the aluminum compound for the coating, the same aluminum compounds asthose described above as the sintering preventive are usable.

The amount of aluminum compound added is usually 0.01 to 50.00 wt %(calculated as Al) based on the weight of the acicular hematiteparticles. When the amount is less than 0.01 wt %, the improvement ofthe dispersibility in the vehicle may be insufficient. On the otherhand, if the amount exceeds 50.00 wt %, the coating effect becomessaturated, so that it is meaningless to add an aluminum compound morethan necessary.

As the silicon compound, the same silicon compounds as those describedabove as the sintering preventive are usable.

The amount of silicon compound added is usually 0.01 to 50.00 wt %(calculated as SiO₂) based on the weight of the acicular hematiteparticles. When the amount is less than 0.01 wt %, the improvement ofthe dispersibility in the vehicle may be insufficient. On the otherhand, when the amount exceeds 50.00 wt %, the coating effect becomessaturated, so that it is meaningless to add an silicon compound morethan necessary.

When both an aluminum compound and a silicon compound are used, theamount of thereof used is preferably 0.01 to 50.00 wt % (calculated asAl and SiO₂) based on the weight of the acicular hematite particles.

It is important in the present invention that when the high-purity andhigh-density acicular hematite particles in which at least a part of thesurface of the particle is coated with an oxide of tin or oxides of tinand antimony, and, which have an average major axial diameter of notmore than 0.3 μm, a pH value of not less than 8, and which containsoluble sodium salts of not more than 300 ppm soluble sodium (calculatedas Na) and soluble sulfates of not more than 150 ppm soluble sulfate(calculated as SO₄), are used as the non-magnetic particles for anon-magnetic undercoat layer, it is possible to enhance the strength andthe surface smoothness of the non-magnetic undercoat layer owing to theexcellent dispersibility of the high-purity and high-density acicularhematite particles into a binder resin; and that when a magneticrecording medium is formed by using the non-magnetic undercoat layer, itis possible to reduce the light transmittance and the surfaceresistivity, to enhance the strength and to make the surface of themagnetic recording layer more smooth. Further, it is possible tosuppress the deterioration in the magnetic. properties which is causedby the corrosion of the acicular magnetic particles containing iron as amain ingredient dispersed in the magnetic recording layer.

The reason why the strength of the non-magnetic undercoat layer isenhanced and the surface of the non-magnetic undercoat layer is mademore smooth, is considered to be as follows. Since it is possible tosufficiently remove the soluble sodium and the soluble sulfate, whichagglomerate hematite particles by firmly crosslinking, by washing theparticles with water, the agglomerates are separated into substantiallydiscrete particles, so that acicular hematite particles having anexcellent dispersion in the vehicle are obtained.

This fact will be explained in the following. The goethite particles asthe starting material are produced by various methods, as describedabove. When as the raw material for producing acicular goethiteparticles ferrous sulfate is used in any method, a large amount ofsulfate [SO₄ ²⁻] naturally exists in the goethite suspension.

Especially, when goethite particles are produced from an acidicsolution, since water-soluble sulfate such as Na₂SO₄ is simultaneouslyproduced and an alkali metal such as K⁺, NH₄ ⁺, and Na⁺ are contained inthe goethite suspension, a deposit containing an alkali metal and asulfate is easily produced. This deposit is represented byRFe₃(SO₄)(OH)₆, (R=K⁺, NH₄ ⁺, or Na⁺). Such a deposit is a slightlysoluble sulfuric acid-containing salt and cannot be removed by anordinary water-washing method. This slightly soluble salt becomes asoluble sodium salt or a soluble sulfate in the next heat-treatmentstep. The soluble sodium salt and soluble sulfate are firmly combinedwith the interiors or the surfaces of the acicular hematite particles bya sintering preventive, which is essential for preventing thedeformation of the acicular hematite particles and sintering betweenparticles in the heat-treatment at a high temperature for thedensification of the particles and which is crosslinking the acicularhematite particles. In this manner, agglomeration between acicularhematite particles becomes further firmer. As a result, the solublesulfate and the soluble sodium salt, especially, imprisoned in theinteriors of the particles or the agglomerates become very difficult toremove by an ordinary water-washing method.

When acicular goethite particles are produced in an aqueous alkalisolution by using ferrous sulfate and sodium hydroxide, Na₂SO₄ issimultaneously produced as a sulfate and NaOH exists in the goethitesuspension. Since they are both soluble, if the acicular goethiteparticles are adequately washed with water, Na₂SO₄ and NaOH ought to beremoved. However, since the crystallinity of acicular goethite particlesis generally small, the water-washing effect is poor, and when theparticles are washed with water by an ordinary method, the particlesstill contain water-soluble contents such as a soluble sulfate [SO₄ ²⁻]and a soluble sodium salt [Na⁺]. The water-soluble contents are firmlycombined with the interiors or the surfaces of the acicular hematiteparticles by the sintering preventive which is crosslinking theparticles, as described above, and the agglomeration between acicularhematite particles becomes further firmer. As a result, the solublesulfate and the soluble sodium salt, especially, imprisoned in theinteriors of the particles or the agglomerates become very difficult toremove by an ordinary water-washing method.

It is considered that when the high-density acicular hematite particlesin which the soluble sodium salt and the soluble sulfate are firmlycombined with the interiors or the surfaces of the particles via thesoluble sintering preventive, as described above, are pulverized by awet-process so as to deagglomerate coarse particles, and heat-treated inthe aqueous alkali solution having a pH value of not less than 13 at atemperature of not less than 80° C., the aqueous alkali solutionsufficiently permeates into the interiors of the hematite particles, sothat the binding strength of the sintering preventive which is firmlycombined with the interiors and the surfaces of the particles, and theinteriors of the agglomerates is gradually weakened, and thecrosslinking is dissociated from the interiors and the surfaces of theparticles and the interiors of the agglomerates, and simultaneously, thewater-soluble sodium salt and the water-soluble sulfate are easilyremoved by water-washing.

It is considered that the deterioration in the magnetic properties whichis caused by the corrosion of the acicular magnetic particles containingiron as a main ingredient, which are dispersed in the magnetic recordinglayer is suppressed because the contents of the soluble sodium salt andthe soluble sulfate, which accelerate the corrosion of a metal, in theacicular hematite particles are small and the pH value of the hematiteparticles themselves is as high as not less than 8.

Actually, it is confirmed that a progress of corrosion of acicularmagnetic particles containing iron as a main ingredient was suppressedby a synergistic effect of a small soluble content and a pH value of notless than 8, from the fact that the advantages of the present inventionwas not attained in any of the cases of (i) heat-treating the hematiteparticles after wet-pulverization in a slurry with the pH value adjustedto less than 13 at a temperature of not less than 80° C., (ii)heat-treating the hematite particles in a slurry with the pH valueadjusted to not less than 13 at a temperature of less than 80° C., or(iii) heat-treating the hematite particles containing coarse particleswithout being pulverized by a wet-process in a slurry with the pH valueadjusted to not less than 13 at a temperature of not less than 80° C.,as shown in later-described examples and comparative examples.

By using the high-density acicular hematite particles according to thepresent invention, a non-magnetic undercoat layer having an excellentsurface smoothness and a uniform thickness because of their excellentdispersibility in vehicle, as described above, can be obtained, and amechanical strength of a substrate when the non-magnetic undercoat layeris formed thereon can be improved. Accordingly, the high-densityacicular hematite particles according to the present invention can besuitably used as non-magnetic particles for non-magnetic undercoatlayer.

Further, by using the non-magnetic undercoat layer according to thepresent invention, it becomes possible to form thereon a magneticrecording layer having an excellent surface smoothness and a uniformthickness due to its excellent properties described above. Accordingly,the non-magnetic undercoat layer according to the present invention canbe suitably used as a non-magnetic undercoat layer of a magneticrecording medium for high-density recording.

Furthermore, the magnetic recording medium according to the presentinvention can exhibit a low transmittance and a low surface resistivity,because the high-density acicular hematite particles used therein havean excellent dispersibility in vehicle, and are coated with an oxide oftin or oxides of tin and antimony. In addition, since the high-densityacicular hematite particles have a less soluble sodium salt content, aless soluble sulfate content and a pH value of not less than 8, acicularmagnetic particles containing iron as a main ingredient , dispersed in amagnetic recording layer of the magnetic recording medium, can beprevented from being corroded, thereby inhibiting the deterioration inmagnetic properties of the magnetic recording layer. Accordingly, themagnetic recording medium according to the present invention canmaintain its excellent properties for a long period of time.

EXAMPLES

The present invention is described in more detail by Examples andComparative Examples, but the Examples are only illustrative and,therefore, not intended to limit the scope of this invention.

Various properties of the high-density acicular particles, non-magneticundercoat layer and magnetic recording medium according to the presentinvention were evaluated by the following methods.

(1) The residue on sieve after the wet-pulverization was obtained bymeasuring the concentration of the slurry after pulverization by awet-process in advance, and determining the quantity of the solidcontent on the sieve remaining after the slurry equivalent to 100 g ofthe solid content was passed through the sieve of 325 meshes (mesh size:44 μm).

(2) The average major axial diameter and the average minor axialdiameter of the particles are expressed by the average values of 350particles measured in the photograph obtained by magnifying an electronmicrograph (×30000) by 4 times in the vertical and horizontaldirections, respectively. The aspect ratio is the ratio of the averagemajor axial diameter and the average minor axial diameter.

(3) The geometrical standard deviation (σg) of particle sizedistribution of the major axial diameter was obtained by the followingmethod. The major axial diameters of the particles were measured fromthe magnified electron microphotograph in the above-mentioned (2). Theactual major axial diameters of the particles and the number ofparticles were obtained from the calculation on the basis of themeasured values. On logarithmico-normal probability paper, the majoraxial diameters were plotted at regular intervals on the abscissa-axisand the accumulative number of particles belonging to each interval ofthe major axial diameters was plotted by percentage on the ordinate-axisby a statistical technique. The major axial diameters corresponding tothe number of particles of 50% and 84.13%, respectively, were read fromthe graph, and the geometrical standard deviation (σg) was measured fromthe following formula:

Geometrical standard deviation (σg)={major axial diameter (μm)corresponding to 84.13% under integration sieve}/{major axial diameter(geometrical average diameter) corresponding to 50% under integrationsieve}.

The smaller the geometrical standard deviation, the more excellent theparticle size distribution of the major axial diameters of theparticles.

(4) The specific surface area is expressed by the value measured by aBET method.

(5) The decree of denseness of the particles is represented byS_(BET)/S_(TEM) as described above. The S_(BET) is a specific surfacearea measured by the above-described BET method. The S_(TEM)is a valuecalculated from the average major axial diameter d cm and the averageminor axial diameter w cm measured from the electron microphotographdescribed in (2) on the assumption that a particle is a rectangularparallellopiped in accordance with the following formula:

S _(TEM)(m²/g)={(4d·w+2w ²)/(d·w ²·ρ_(p))}×10⁻⁴

wherein ρ_(p) is the true specific gravity of the hematite particles,and 5.2 g/cm³ was used.

Since S_(TEM) is a specific surface area of a particle having a smoothsurface without any dehydration pore within or on the surface thereof,the closer S_(BET)/S_(TEM)of particles is to 1, it means, the smoothersurface the particles have without any dehydration pore within or in thesurface thereof, in other words, the particles are high-densityparticles.

(6) The content of each of Sn, Sb, Al, Co, P and Si was measured fromfluorescent X-ray analysis.

(7) The pH value of the particles was measured in the following method.5 g of the sample was weighed into a 300-ml triangle flask, and 100 mlof pure water was added. The suspension was heated and after keeping theboiled state for 5 minutes, it was corked and left to cool to anordinary temperature. After adding pure water which was equivalent tothe pure water lost by boiling, the flask was corked again, shaken for 1minute, and left to stand for 5 minutes. The pH value of the supernatantobtained was measured in accordance JIS Z 8802-7.

(8) The contents of soluble sodium salts and soluble sulfates weremeasured by measuring the Na content and SO₄ ²-content in the filtrateobtained by filtering the supernatant liquid produced for themeasurement of pH value which is described above through filter paperNo. 5C, by using an Inductively Coupled Plasma EmissionSpectrophotometer (manufactured by Seiko Instruments and Electronics,Ltd.).

(9) The volume resistivity of acicular hematite particles was measuredas follows. First, 0.5 g of acicular hematite particles were weighed andpressure-molded under 140 kg/cm² by KBr pellet molding apparatus(manufactured by Simazu Seisakusho Co., Ltd.) to form a cylindricalsample.

Next, the cylindrical sample was allowed to stand at a temperature of25° C. and a relative humidity (RH) of 60% for not less than 12 hours.The sample was set between stainless steel electrodes, and impressedwith a voltage of 15 V using a Wheatstone bridge “TYPE 2768”(manufactured by Yokogawa Hokushin Denki Co., Ltd.) to measure aresistance (Ω) thereof.

Next, the sample was measured for a upper surface area A (cm²) and athickness t (cm) thereof. A volume resistivity X (Ω·cm) is obtained bythe following formula:

X(∩.cm)=R×(A/t)

wherein R represents an actual measured value of resistance.

(10) The surface resistivity of a coating film was measured as follows.The coating film was first allowed to stand at a temperature of 25° C.and a relative humidity (RH) of 60% for not less than 12 hours.Thereafter, the coating film which was slit into a width of 6 mm, andwas disposed between metal electrodes each having a width of 6.5 mm,such that a coating surface of the coating film was contacted with theseelectrodes. 170 g of weights were respectively fixed to opposite ends ofthe coating film so as to bring the coating surface of the coating filminto close contact with the electrodes. Thereafter, D.C. voltage of 500V was applied between the metal electrodes to measure a surfaceresistivity of the coating film by using a resistance meter Model 14329A(manufactured by Yokogawa Hewlett Packard Co., Ltd.).

(11) The viscosity of the coating composition was obtained by measuringthe viscosity of the coating composition at 25° C. at a shear rate D of1.92 sec⁻¹ by using an E type viscometer EMD-R (manufactured by TokyoKeiki, Co., Ltd.).

(12) The gloss of the surface of the coating film of each of thenon-magnetic undercoat layer and the magnetic recording layer wasmeasured at an angle of incidence of 45° by a glossmeter UGV-5D(manufactured by Suga Shikenki, Co., Ltd.).

(13) The surface roughness Ra is expressed by the average value of thecenter-line average roughness of the profile curve of the surface of thecoating film by using “Surfcom-575A” (manufactured by Tokyo SeimitsuCo., Ltd.).

(14) The strength of the non-magnetic undercoat layer and magneticrecording medium was expressed the Young's modulus obtained by“Autograph” (produced by Shimazu Seisakusho Ltd.). The Young's moduluswas expressed by the ratio of the Young's modulus of the coating film tothat of a commercially available video tape “AV T-120” (produce byVictor Company of Japan, Ltd.). The higher the relative value, the morefavorable.

(15) The magnetic Properties were measured under an external magneticfield of 10 kOe by “Vibration Sample Magnetometer VSM-3S-15”(manufactured by Toei Kogyo, Co., Ltd.).

(16) The chance in the magnetic properties with Passage of time of amagnetic recording medium caused by the corrosion of the magneticparticles containing iron as a main ingredient was examined as follows.

The magnetic recording medium was allowed to stand in an environment ofa temperature of 60° C. and a relative humidity of 90% for 14 days, andthe coercive force and the saturation magnetic flux density weremeasured before and after standing. A change in each characteristic wasdivided by the value before standing, and represented by percentage as apercentage of change.

(17) The light transmittance of a magnetic recording medium is expressedby the linear adsorption coefficient using a light transmittance atλ=900 nm measured by “Photoelectric Spectrophotometer UV-2100”(manufactured by Shimazu Seisakusho, Ltd.). The linear adsorptioncoefficient is defined by the following formula:

Linear adsorption coefficient (μm⁻¹)={1 n(1/t)}/FT

wherein t represents light transmittance (−) at λ=900 nm, and FTrepresents thickness (μm) of the coating film composition of the filmused for the measurement.

The larger the value, the more difficult it is for the magneticrecording medium to transmit light.

As a blank for measuring the linear adsorption coefficient, the samenon-magnetic substrate as that of the above-mentioned magnetic recordingmedium, was used.

(18) The thickness of each of the non-magnetic substrate, thenon-magnetic undercoat layer and the magnetic recording layerconstituting the magnetic recording medium was measured in the followingmanner by using a Digital Electronic Micrometer K351C (manufactured byAnritsu Denki Corp.)

The thickness (A) of a non-magnetic substrate was first measured.Similarly, the thickness (B) (B=the sum of the thicknesses of thenon-magnetic substrate and the non-magnetic undercoat layer) of asubstrate obtained by forming a non-magnetic undercoat layer on thenon-magnetic substrate was measured. Furthermore, the thickness (C)(C=the sum of the thicknesses of the non-magnetic substrate, thenon-magnetic undercoat layer and the magnetic recording layer) of amagnetic recording medium obtained by forming a magnetic recording layeron the non-magnetic undercoat layer was measured. The thickness of thenon-magnetic undercoat layer is expressed by B−A, and the thickness ofthe magnetic recording layer is expressed by C−B.

Example 1 Production of Acicular Hematite Particles

Example 1

1,200 g of spindle-shaped goethite particles obtained by theafore-mentioned goethite production method (B) (average major axialdiameter: 0.178 μm, average minor axial diameter: 0.0225 μm, aspectratio: 7.91:1, BET specific surface area: 160.3 m²/g, soluble sodiumsalt content: 1232 ppm (calculated as Na), soluble sulfate content: 621ppm (calculated as SO₄), pH value: 6.7 and geometrical standarddeviation: 1.33) were suspended in a mixed solution of an aqueousferrous sulfate solution and an aqueous sodium carbonate solution toform a slurry having a solid content of 8 g/liter. After 150 liters ofthe slurry was heated to 60° C., a 0.1N NaOH aqueous solution was addedthereto to adjust the pH value to 9.0.

Next, 2,022 ml of an aqueous solution containing 0.5 mol/liter of sodiumstannate was gradually added to the thus obtained alkaline slurry. Aftercompletion of the addition, a 0.8N acetic acid solution was added to theslurry to adjust the pH value to 7.5. Thereafter, the slurry wassuccessively filtered, washed with water, dried and pulverized byordinary methods to obtain spindle-shaped goethite particles whosesurfaces were coated with a hydroxide of tin. It was confirmed that theamount of the hydroxide of tin was 9.32% by weight (calculated as Sn)based on the weight of the spindle-shaped goethite particles.

1,000 g of the thus obtained spindle-shaped goethite particles coatedwith the hydroxide of tin, were charged into a stainless steel rotaryfurnace, and heat-dehydrated in air at 350° C. for 60 minutes whilerotating the furnace, thereby obtaining low-density spindle-shapedhematite particles coated with an oxide of tin. It was determined thatthe thus obtained low-density spindle-shaped hematite particles had anaverage major axial diameter of 0.134 μm, an average minor axialdiameter of 0.0194 μm, an aspect ratio (average major axialdiameter/average minor axial diameter) of 6.91:1, a BET specific surfacearea (S_(BET)) of 168.3 m²/g, a degree of densification(S_(BET)/S_(TEM)) of 3.96, a soluble sodium salt content of 1123 ppm(calculated as Na), a soluble sulfate content of 465 ppm (calculated asSO₄), a pH value of 6.3, a geometrical standard deviation of 1.34 and avolume resistivity of 1.1×10⁶ Ωcm. Further, the amount of the oxide oftin was 10.60% by weight (calculated as Sn) based on the weight of thespindle-shaped hematite particles.

Next, 900 g of the low-density spindle-shaped hematite particles werecharged into a ceramic rotary furnace, and heated in air at 650° C. for20 minutes while rotating the furnace to seal dehydrating pores of theparticles, thereby obtaining high-density spindle-shaped hematiteparticles coated with the oxide of tin. It was determined that theobtained high-density spindle-shaped hematite particles had an averagemajor axial diameter of 0.129 μm, an average minor axial diameter of0.0206 μm, an aspect ratio (average major axial diameter/average minoraxial diameter) of 6.26:1, a BET specific surface area (S_(BET)) of 46.6m²/g, a degree of high densification (S_(BET)/S_(TEM)) of 1.16, asoluble sodium salt content of 2864 ppm (calculated as Na), a solublesulfate content of 2956 ppm (calculated as SO₄), a pH value of 5.4, ageometrical standard deviation of 1.36 and a volume resistivity of9.6×10⁵ Ωcm. Further, the amount of the oxide of tin was 10.72% byweight (calculated as Sn) based on the weight of the spindle-shapedhematite particles.

After 800 g of the high-density spindle-shaped hematite particlesobtained were roughly pulverized by a Nara mill in advance, the obtainedparticles were charged into 4.7 liters of pure water and peptized by ahomomixer (manufactured by Tokushu-kika Kogyo, CO., Ltd.) for 60minutes.

The slurry of the high-density spindle-shaped hematite particlesobtained was then mixed and dispersed for 3 hours at an axial rotationfrequency of 2000 rpm while being circulated by a horizontal SGM(Dispermat SL, manufactured by S.C. Adichem, CO., Ltd.). Thespindle-shaped hematite particles in the slurry remaining on a sieve of325 meshes (mesh size: 44 μm) was 0% by weight.

The concentration of the high-density spindle-shaped hematite particlesin the slurry was adjusted to 100 g/liter, and a 6N-aqueous NaOHsolution was added to 7 liter of the slurry under stirring so as toadjust the pH value to 13.3. The slurry was then heated to 95° C. understirring, and was held for 3 hours at 95° C.

The slurry was then washed with water by a decantation method and the pHvalue of the slurry was adjusted to 10.5. When the concentration of theslurry at this point was checked so as to ensure the accuracy, it was 96g/liter.

2 liter of the slurry washed with water was filtered through a Buchnerfilter, and pure water was passed until the electric conductivity of thefiltrate became not more than 30 μs. The particles were then dried by anordinary method and pulverized so as to obtain the target high-densityspindle-shaped hematite particles. The high-density spindle-shapedhematite particles obtained contained 10.83 wt % of an oxide of tin(calculated as Sn), and had an average major axial diameter of 0.128 μm,a minor axial diameter of 0.0206 μm, an aspect ratio of 6.71:1, ageometric standard deviation σg of particle size (major axial diameter)of 1.35, a BET specific surface (S_(BET)) of 47.1 m²/g, aS_(BET)/S_(TEM) value of densification of 1.17 and a pH value of theparticles of 8.9. The spindle-shaped hematite particles containedsoluble sodium salts of 112 ppm soluble sodium (calculated as Na) andsoluble sulfates of 41 ppm soluble sulfate (calculated as SO₄). Thevolume resistivity thereof was 6.3×10⁶ Ωcm.

Example 2 Production of a Non-magnetic Undercoat Layer

12 g of the high-density spindle-shaped hematite particles obtained inthe Example 1 were mixed with a binder resin solution (30 wt % of vinylchloride-vinyl acetate copolymer resin having a sodium sulfonate groupand 70 wt % of cyclohexanone) and cyclohexanone, and the mixture (solidcontent: 72 wt%) obtained was kneaded by a plasto-mill for 30 minutes.

The thus-obtained kneaded material was charged into a 140 ml-glassbottle together with 95 g of 1.5 mmφ glass beads, a binder resinsolution (30 wt % of polyurethane resin having a sodium sulfonate groupand 70 wt % of a solvent (methyl ethyl ketone:toluene=1:1)),cyclohexanone, methyl ethyl ketone and toluene, and the obtained mixturewas mixed and dispersed by a paint shaker for 6 hours to obtain acoating composition.

The thus-obtained coating composition containing high-densityspindle-shaped hematite particles was as follows:

High-density spindle-shaped 100 parts by weight hematite particles Vinylchloride-vinyl acetate 10 parts by weight copolymer resin having asodium sulfonate group Polyurethane resin having a 10 parts by weightsodium sulfonate group Cyclohexanone 44.6 parts by weight Methylethylketone 111.4 parts by weight Toluene 66.9 parts by weight

The coating composition obtained containing high-density spindle-shapedhematite particles was applied to a polyethylene terephthalate film of12 μm thick to a thickness of 55 μm by an applicator, and the film wasthen dried, thereby forming a non-magnetic undercoat layer. Thethickness of the non-magnetic undercoat layer was 3.5 μm.

The gloss of the coating film of the obtained non-magnetic undercoatlayer was 201%, the surface roughness Ra was 6.8 nm, and the Young'smodulus (relative value) was 128.

Example 3 Production of a Magnetic Recording Layer

12 g of spindle-shaped magnetic particles containing iron as a mainingredient (average major axial diameter: 0.104 μm, average minor axialdiameter: 0.0158 μm, aspect ratio: 6.58:1, coercive force: 1905 Oe,saturation magnetization: 138 emu/g, Al content: 4.41 wt %, and Cocontent: 5.51 wt %), 1.2 g of a polishing agent (AKP-50: trade name,produced by Sumitomo Chemical Co., Ltd.), 0.36 g of carbon black(#3250B, trade name, produced by Mitsubishi Chemical Corp.), a binderresin solution (30 wt % of vinyl chloride-vinyl acetate copolymer resinhaving a sodium sulfonate group and 70 wt % of cyclohexanone) andcyclohexanone were mixed to obtain a mixture (solid content: 78 wt%).The mixture was further kneaded by a plasto-mill for 30 minutes toobtain a kneaded material.

The thus-obtained kneaded material was charged into a 140 ml-glassbottle together with 95 g of 1.5 mmφ glass beads, a binder resinsolution (30 wt % of polyurethane resin having a sodium sulfonate groupand 70 wt % of a solvent (methyl ethyl ketone:toluene=1:1)),cyclohexanone, methyl ethyl ketone and toluene, and the mixture wasmixed and dispersed by a paint shaker for 6 hours. Thereafter, thelubricant and hardening agent were added to the resultant mixture, andthen the obtained mixture was mixed and dispersed by paint shaker for 15minutes.

The thus-obtained magnetic coating composition was as follows:

Magnetic particles containing 100 parts by weight iron as a mainingredient Vinyl chloride-vinyl acetate 10 parts by weight copolymerresin having a sodium sulfonate group Polyurethane resin having a 10parts by weight sodium sulfonate group Polishing agent (AKP-50) 10 partsby weight Carbon black (#3250B) 3.0 parts by weight Lubricant (myristicacid:butyl 3.0 parts by weight stearate = 1:2) Hardening agent(polyisocyanate) 5.0 parts by weight Cyclohexanone 65.8 parts by weightMethyl ethyl ketone 164.5 parts by weight Toluene 98.7 parts by weight

The magnetic coating composition obtained was applied to thenon-magnetic undercoat layer to a thickness of 15 μm by an applicator,and the magnetic recording medium obtained was oriented and dried in amagnetic field, and then calendered. The magnetic recording medium wasthen subjected to a curing reaction at 60° C. for 24 hours, andthereafter slit into a width of 0.5 inch, thereby obtaining a magnetictape. The thickness of the magnetic recording layer was 1.1 μm.

The magnetic tape obtained had a coercive force of 1986 Oe, a squareness(Br/Bm) of 0.87, a gloss of 228%, a surface roughness Ra of 6.4 nm, aYoung's modulus (relative value) of 133, a linear absorption coefficientof 1.21, a surface resistivity of 1.1×10⁷ ω/sq. The changes in thecoercive force and the saturation magnetic flux density Bm with passageof time were 6.4%, and 5.4%, respectively.

Examples 4 to 21, Comparative Examples 1 to 14 Types of AcicularGoethite Particles

The starting materials A to F shown in Table 1 were used as the startingmaterials for producing acicular hematite particles.

Production of Low-density Acicular Hematite Particles

Low-density acicular hematite particles were obtained in the same way asin Example 1 except for varying the kind of acicular goethite particlesas the starting materials, the kind and amount of tin compound, the kindand amount of antimony compound, the kind and amount of sinteringpreventive, and heat-dehydrating temperature and time.

The main producing conditions and various properties are shown in Tables2 to 3.

Examples 22 to 39, Comparative Examples 15 to 27 Production ofHigh-density Acicular Hematite Particles

High-density acicular hematite particles were obtained in the same wayas in Example 1 except for varying the kind of low-density hematiteparticles, and the heating temperature and time for densification.

The main producing conditions and various properties are shown in Tables4 and 5.

Examples 40 to 57, Comparative Examples 28 to 35 Treatment of AcicularHematite Particles in an Aqueous Alkali Solution

High-purity, high-density acicular hematite particles were obtained inthe same way as in Example 1 except for varying the kind of high-densityacicular hematite particles, whether or not the wet-pulverizationprocess was conduced, whether or not the heat-treatment in the aqueousalkali solution was conducted, the pH value of the slurry, and theheating time and temperature.

The main producing conditions and various properties are shown in Tables6 to 9.

Example 58 Surface Coating of Acicular Hematite Particles

The concentration of the slurry having a pH value 10.5 which wasobtained in Example 40 by washing the particles in an aqueous alkalisolution after heat-treatment with water by a decantation method was 96g/liter. 5 liter of the slurry was re-heated to 60° C., and 231 ml(equivalent to 1.3 wt % (calculated as Al) based on the acicularhematite particles) of a 1.0-N NaAlO₂ solution was added to the slurry,and the mixture was held for 60 minutes. Thereafter, the pH value of themixture was adjusted to 8.2 by using acetic acid. The particles werethen filtered out, washed with water, dried and pulverized in the sameway as in Example 1, thereby obtaining acicular hematite particlescoated with a coating material.

The main producing conditions and various properties are shown in Tables10 and 11.

Examples 59 to 72

Acicular hematite particles coated with a coating material were obtainedin the same way as in Example 58 except for varying the kind of acicularhematite particles, and the kind and the amount of surface treatingmaterial.

The main producing conditions and various properties are shown in Table10 and 11.

Example 73 Coating-treatment of High-density Acicular Hematite ParticlesTreated with Alkaline Aqueous Solution, with Oxide of Tin or Oxides ofTin and Antimony

The slurry obtained in Example 55 by washing with water by a decantationmethod after the heat-treatment in alkaline aqueous solution, had a pHvalue of 10.5 and a concentration of 96 g/liter. After 5 liters of theslurry was heated again to 60° C., 121 ml of a 1.0-mol sodium stannatesolution (corresponding to 3.0% by weight (calculated as Sn) based onthe weight of the acicular hematite particles) was added thereto. Afterthe slurry was allowed to stand for 60 minutes, the pH value thereof wasadjusted to 8.0 by adding acetic acid thereto. Next, the slurry wasfiltered to separate a solid component therefrom, and then the solidcomponent was washed with water, dried and pulverized in the same manneras in Example 1, thereby obtaining high-density acicular hematiteparticles coated with a hydroxide of tin.

The essential production conditions and properties of the obtainedhigh-density acicular hematite particles are shown in Tables 10 and 11.

Examples 74 and 75

The same procedure as defined in Example 73 was conducted except thatkind of acicular hematite particles treated with alkaline aqueoussolution, kind and amount of tin compound, kind, amount and use ornon-use of antimony compound and treating temperature and treating timeused for the heat-treatment were varied, thereby obtaining high-densityacicular hematite particles coated with a hydroxide of tin or hydroxidesof tin and antimony.

The essential production conditions and properties of the obtainedhigh-density acicular hematite particles are shown in Tables 10 and 11.

Example 76

The high-density acicular hematite particles coated with the hydroxideof tin which were obtained in Example 73, were charged into a stainlesssteel rotary furnace, and heated in air at 400° C. for 60 minutes whilerotating the furnace, thereby obtaining high-density acicular hematiteparticles coated with an oxide of tin.

The essential production conditions and properties of the obtainedhigh-density acicular hematite particles are shown in Tables 12 and 13.

Examples 77 and 78

The same procedure as defined in Example 76 was conducted except thatkind of high-density acicular hematite particles coated with a hydroxideof tin or hydroxides of tin and antimony and treating temperature andtreating time used for the heat-treatment were varied, thereby obtaininghigh-density acicular hematite particles coated with an oxide of tin oroxides of tin and antimony.

The essential production conditions and properties of the obtainedhigh-density acicular hematite particles are shown in Tables 12 and 13.

Examples 79 to 111, Comparative Examples 36 to 50 Production of aNon-magnetic Undercoat Layer

A non-magnetic undercoat layer was obtained in the same way as inExample 2 by using the acicular hematite particles obtained in Examples40 to 54, 58 to 72 and 76 to 78, Comparative Examples 1, 3, 15 to 18, 23and 28 to 35.

The main producing conditions and various properties are shown in Tables14 to 16.

Examples 112 to 144, Comparative Examples 51 to 65 Production of aMagnetic Recording Medium Using Magnetic Particles Containing Iron as aMain Ingredient

A magnetic recording medium using acicular magnetic particles containingiron as a main ingredient was obtained in the same way as in Example 3except for varying the kind of non-magnetic undercoat layer obtained inExamples 79 to 111 and Comparative Examples 36 to 50 and the kind ofacicular magnetic particles containing iron as a main ingredient.

The main producing conditions and various properties are shown in Tables17 to 19.

TABLE 1 Acicular Goethite particles Kind of Average major Average minorstarting Production axial axial material method diameter (μm) diameter(μm) Starting BB 0.181 0.0246 material A Starting BB 0.220 0.0283material B Starting DD 0.245 0.0305 material C Starting CC 0.164 0.0218material D Starting AA 0.260 0.0298 material E Starting BB 0.234 0.0288material F Acicular Goethite particles Geometrical Kind of standard BETspecific starting Aspect ratio* deviation σg surface area material (−)(−) (m²/g) Starting 7.36 1.37 151.0 material A Starting 7.77 1.34 125.0material B Starting 8.03 1.31 95.1 material C Starting 7.52 1.37 186.5material D Starting 8.72 1.44 72.6 material E Starting 8.13 1.31 110.5material F Acicular Goethite particles Kind of Soluble starting sodiumsalt Soluble pH value material (ppm) sulfate (ppm) (−) Starting 412 3866.8 material A Starting 512 264 7.2 material B Starting 1215 2150 5.1material C Starting 415 915 5.5 material D Starting 1565 171 8.3material E Starting 436 312 6.9 material F (Note) *Aspect ratio =average major axial diameter/average minor axial diameter (Note)PRODUCTION METHOD: AA: A method of oxidizing a suspension having a pHvalue of not less than 11 and containing colloidal ferrous hydroxideparticles which is obtained by adding not less than an equivalent of analkali hydroxide solution to an aqueous ferrous salt solution, bypassing an oxygen-containing gas thereinto at a temperature of nothigher than 80° C. BB: A method of producing acicular goethite particlesby oxidizing a suspension containing FeCO₃ which is obtained by reactingan aqueous ferrous salt solution with an aqueous alkali carbonatesolution, by passing an oxygen-containing gas thereinto after aging thesuspension, if necessary. CC: A method of growing acicular seed goethiteparticles by oxidizing a ferrous hydroxide solution containing colloidalferrous hydroxide particles which is obtained by adding less than anequivalent of an alkali hydroxide solution or an alkali carbonatesolution to an aqueous ferrous salt solution, by passing anoxygen-containing gas thereinto, thereby producing acicular seedgoethite particles, adding not less than an equivalent of an alkalihydroxide solution to the #Fe²⁺ in the aqueous ferrous salt solution, tothe aqueous ferrous salt solution containing the acicular goethite seedparticles, and passing an oxygen-containing gas into the aqueous ferroussalt solution. DD: A method of growing acicular seed goethite particlesby oxidizing a ferrous hydroxide solution containing colloidal ferroushydroxide particles which is obtained by adding less than an equivalentof an alkali hydroxide solution or an alkali carbonate solution to anaqueous ferrous salt solution, by passing an oxygen-containing gasthereinto, thereby producing acicular seed goethite particles, andgrowing the obtained acicular seed goethite particles in an acidic orneutral region.

TABLE 2 Kind of acicular Sintering preventive goethite particles Cal-Amount as the starting culated added Examples material Kind as (wt. %)Example 4 Particles of Sodium stannate Sn 15.0 Example 1 Example 5Starting material A Stannous chloride Sn 20.0 Example 6 Startingmaterial A Sodium stannate Sn 50.0 #3 Water glass SiO₂  1.0 Example 7Starting material B Sodium stannate Sn  1.2 Example 8 Starting materialB Sodium stannate Sn  3.0 Example 9 Starting material C Stannouschloride Sn 10.0 Example 10 Starting material C Sodium stannate Sn 50.0Example 11 Starting material D Sodium stannate Sn 200.0  Example 12Starting material D Sodium stannate Sn 500.0  Example 13 Startingmaterial E Stannous chloride Sn 100.0  Example 14 Starting material ESodium stannate Sn 75.0 Phosphoric acid P  0.5 Example 15 Startingmaterial F Sodium stannate Sn 50.0 Antimony Sb  5.0 chloride Example 16Starting material F Sodium stannate Sn 10.0 Antimony acetate Sb  2.0Example 17 Particles of Sodium stannate Sn 100.0  Example 1 Antimony solSb 10.0 #3 Water glass SiO₂ 1.5 Example 18 Particles of Stannouschloride Sn 300.0  Example 1 Antimony Sb 15.0 chloride Aluminum sulfateAl  1.0 Example 19 Starting material A Sodium P  1.5 hexameta- phosphateExample 20 Starting material A #3 Water glass SiO₂  2.0 Example 21Starting material A Phosphoric acid P  0.5 #3 Water glass SiO₂  0.75Low-density acicular hematite Heat treatment for low particlesdensification Average major Average minor Temperature Time axialdiameter axial diameter Examples (° C.) (min) (μm) (μm) Example 4 300 900.138 0.0199 Example 5 350 30 0.143 0.0214 Example 6 330 120  0.1400.0221 Example 7 280 30 0.180 0.0261 Example 8 300 60 0.183 0.0260Example 9 330 120  0.200 0.0277 Example 10 350 90 0.206 0.0290 Example11 380 60 0.138 0.0211 Example 12 400 30 0.140 0.0240 Example 13 380 600.218 0.0283 Example 14 380 90 0.213 0.0276 Example 15 350 60 0.1910.0277 Example 16 375 60 0.192 0.0278 Example 17 310 240  0.142 0.0205Example 18 350 180  0.145 0.0211 Example 19 330 60 0.141 0.0213 Example20 310 90 0.141 0.0210 Example 21 350 120  0.144 0.0278 Low-densityacicular hematite particles Geometrical standard Aspect ratio* S_(BET)S_(TEM) Examples deviation σg (−) (−) (m²g) (m²/g) Example 4 1.33 6.93169.1 41.4 Example 5 1.37 6.68 168.5 38.6 Example 6 1.38 6.33 160.8 37.6Example 7 1.34 6.90 144.0 31.6 Example 8 1.34 7.04 135.9 31.7 Example 91.33 7.22 119.0 29.7 Example 10 1.32 7.10 125.1 28.4 Example 11 1.376.54 197.5 39.2 Example 12 1.40 5.83 213.5 34.8 Example 13 1.44 7.70110.8 28.9 Example 14 1.44 7.72 103.6 29.7 Example 15 1.32 6.90 146.929.8 Example 16 1.32 6.91 138.5 29.7 Example 17 1.34 6.93 173.8 40.2Example 18 1.34 6.87 180.6 39.1 Example 19 1.36 6.62 171.6 38.8 Example20 1.36 6.71 178.8 39.4 Example 21 1.36 6.92 186.2 39.7 Low-densityacicular hematite particles S_(BET)/S_(TEM) Soluble sodium Solublesulfate pH value Examples (−) salt (ppm) (ppm) (−) Example 4 4.08 1263680 7.8 Example 5 4.36 1897 879 7.1 Example 6 4.28 1835 980 8.1 Example7 4.56  870 789 6.5 Example 8 4.29 1123 987 6.8 Example 9 4.01 23561234  6.3 Example 10 4.41 2987 1145  6.8 Example 11 5.03 3456 789 7.9Example 12 6.14 4890 891 8.0 Example 13 3.83 2156 1348  6.5 Example 143.49 1768 1123  7.5 Example 15 4.93 1345 888 6.0 Example 16 4.67 18791334  6.1 Example 17 4.32 2350 1345  8.1 Example 18 4.62 2879 789 7.3Example 19 4.42 2006 912 7.5 Example 20 4.54 2128 986 7.3 Example 214.70 2166 982 7.3 (Note) *Aspect ratio average major axialdiameter/average minor axial diameter

TABLE 3 Kind of Sintering preventive acicular goethite Calcu- AmountComparative particles as the lated added Examples starting particlesKind as (wt. %) Comparative Particles of — — — Example 1 Example 1Comparative Particles of — — — Example 2 Example 1 Comparative Particlesof #3 Water glass SiO₂ 0.50 Example 3 Example 1 Comparative Particles ofPhosphoric acid P 0.50 Example 4 Example 1 Comparative Particles of #3Water glass SiO₂ 1.00 Example 5 Example 1 Comparative Particles ofSodium P 0.50 Example 6 Example 1 hexametaphosphate ComparativeParticles of #3 Water glass SiO₂ 1.50 Example 7 Example 1 ComparativeParticles of #3 Water glass SiO₂ 0.20 Example 8 Example 1 ComparativeParticles of Phosphoric acid P 0.75 Example 9 Example 1 ComparativeStarting Sodium P 2.00 Example 10 material F hexametaphosphateComparative Starting #3 Water glass SiO₂ 1.25 Example 11 material FComparative Starting Phosphoric acid P 1.50 Example 12 material FComparative Starting Colloidal silica SiO₂ 0.25 Example 13 material FComparative Starting Sodium stannate Sn 0.05 Example 14 material FLow-density acicular hematite Heat treatment for low particlesdensification Average major Average minor Comparative Temperature Timeaxial diameter axial diameter Examples (° C.) (min) (μm) (μm)Comparative 350 90 0.135 0.0199 Example 1 Comparative 380 45 0.1320.0206 Example 2 Comparative 340 75 0.136 0.0197 Example 3 Comparative —— — — Example 4 Comparative 350 60 0.134 0.0199 Example 5 Comparative350 30 0.132 0.0197 Example 6 Comparative 330 90 0.134 0.0197 Example 7Comparative 300 60 0.132 0.0190 Example 8 Comparative 380 20 0.1320.0195 Example 9 Comparative 380 90 0.193 0.0276 Example 10 Comparative350 90 0.193 0.0280 Example 11 Comparative 330 30 0.192 0.0278 Example12 Comparative 325 45 0.189 0.0288 Example 13 Comparative 300 60 0.1850.0293 Example 14 Low-density acicular hematite particles GeometricalComparative standard Aspect ratio* S_(BET) S_(TEM) Examples deviation σg(−) (−) (m²/g) (m²/g) Comparative 1.33 6.78 171.6 41.5 Example 1Comparative 1.34 6.41 135.8 40.3 Example 2 Comparative 1.33 6.90 165.841.9 Example 3 Comparative — — — — Example 4 Comparative 1.34 6.73 134.841.5 Example 5 Comparative 1.35 6.70 125.9 42.0 Example 6 Comparative1.35 6.80 145.0 41.9 Example 7 Comparative 1.35 6.95 145.9 43.4 Example8 Comparative 1.36 6.77 156.9 42.4 Example 9 Comparative 1.33 6.99 124.329.9 Example 10 Comparative 1.33 6.89 131.2 29.5 Example 11 Comparative1.32 6.91 138.2 29.7 Example 12 Comparative 1.33 6.56 136.3 28.7 Example13 Comparative 1.32 6.31 126.4 28.3 Example 14 Low-density acicularhematite particles Soluble Soluble Volume Comparative S_(BET)/S_(TEM)sodium salt sulfate pH value resistivity Examples (−) (ppm) (ppm) (−)(Ωcm) Comparative 4.13  852 568 6.0 1.2 × 10⁸ Example 1 Comparative 3.37 903 498 6.5 — Example 2 Comparative 3.96 1245 423 6.8 4.0 × 10⁸ Example3 Comparative — — — — — Example 4 Comparative 3.25 1235 568 6.7 —Example 5 Comparative 3.00 1025 612 6.6 — Example 6 Comparative 3.461365 682 7.1 — Example 7 Comparative 3.36 1265 591 7.2 — Example 8Comparative 3.70 1124 654 7.1 — Example 9 Comparative 4.16 1026 689 6.9— Example 10 Comparative 4.45 1176 563 7.1 — Example 11 Comparative 4.661015 597 7.0 — Example 12 Comparative 4.74  892 498 6.2 — Example 13Comparative 4.46 1235 569 7.3 — Example 14 (Note) *Aspect ratio =average major axial diameter/average minor axial diameter

TABLE 4 High-density acicular hematite particles Kind of low- Heattreatment for high Average Average density acicular densification majoraxial minor axial hematite Tempera- diameter diameter Examples particlesture (° C.) Time (min) (μm) (μm) Example 22 Example 4 700 60 0.1320.0210 Example 23 Example 5 730 60 0.138 0.0223 Example 24 Example 6 75060 0.138 0.0230 Example 25 Example 7 600 15 0.169 0.0301 Example 26Example 8 610 15 0.173 0.0288 Example 27 Example 9 650 30 0.186 0.0294Example 28 Example 10 730 90 0.204 0.0315 Example 29 Example 11 750 120 0.l37 0.0222 Example 30 Example 12 800 30 0.139 0.0243 Example 31Example 13 750 30 0.211 0.0299 Example 32 Example 14 750 60 0.208 0.0296Example 33 Example 15 730 60 0.189 0.0285 Example 34 Example 16 710 450.189 0.0298 Example 35 Example 17 750 30 0.139 0.0220 Example 36Example 18 780 60 0.140 0.0219 Example 37 Example 19 680 60 0.141 0.0213Example 38 Example 20 700 30 0.140 0.0211 Example 39 Example 21 700 600.142 0.0210 High-density acicular hematite particles Geometricalstandard Aspect S_(BET) S_(BET)/S_(TEM) Examples deviation σg (−) ratio*(−) (m²/g) S_(TEM) (m²/g) (−) Example 22 1.35 6.29 51.3 39.5 1.30Example 23 1.38 6.19 46.8 37.3 1.26 Example 24 1.38 6.00 44.9 36.2 1.24Example 25 1.35 5.61 38.5 27.8 1.38 Example 26 1.35 6.01 35.1 28.9 1.21Example 27 1.35 6.33 33.7 28.2 1.19 Example 28 1.34 6.48 34.9 26.3 1.33Example 29 1.39 6.17 51.0 37.5 1.36 Example 30 1.41 5.72 53.2 34.4 1.55Example 31 1.44 7.06 37.5 27.5 1.36 Example 32 1.45 7.03 38.9 27.8 1.40Example 33 1.33 6.63 40.1 29.0 1.38 Example 34 1.34 6.34 37.5 27.8 1.35Example 35 1.36 6.32 55.9 37.7 1.48 Example 36 1.35 6.39 57.1 37.9 1.51Example 37 1.36 6.62 51.2 38.8 1.32 Example 38 1.36 6.64 50.6 39.2 1.29Example 39 1.36 6.76 51.6 39.3 1.31 High-density acicular hematiteparticles Kind of Sintering preventive Calculated Amount CalculatedAmount Calculated Amount Examples as (wt. %) as (wt. %) as (wt. %)Example 22 Sn 14.33 — — — — Example 23 Sn 18.06 — — — — Example 24 Sn36.83 — — SiO₂ 0.62 Example 25 Sn  1.30 — — — — Example 26 Sn  3.16 — —— — Example 27 Sn 10.06 — — — — Example 28 Sn 35.81 — — — — Example 29Sn 73.31 — — — — Example 30 Sn 91.68 — — — — Example 31 Sn 55.68 — — — —Example 32 Sn 47.15 P 0.48 — — Example 33 Sn 37.13 Sb 5.14 — — Example34 Sn  9.65 Sb 2.01 — — Example 35 Sn 49.30 Sb 8.88 SiO₂ 1.41 Example 36Sn 80.61 Sb 14.31  Al 0.96 Example 37 — — P 1.36 — — Example 38 — — — —SiO₂ 1.83 Example 39 — — P 0.51 SiO₂ 0.70 High-density acicular hematiteparticles Soluble sodium salt Soluble sulfate pH value Examples (ppm)(ppm) (−) Example 22 1894 3400 5.1 Example 23 2561 3604 4.6 Example 242569 3448 5.7 Example 25 1205 3698 5.6 Example 26 1640 3948 5.2 Example27 3063 3702 4.7 Example 28 3764 3591 5.8 Example 29 4182 3077 6.5Example 30 5477 3475 6.8 Example 31 2911 4044 4.1 Example 32 2581 39314.5 Example 33 2219 2664 5.0 Example 34 2743 4669 4.1 Example 35 34852356 7.8 Example 36 3023 2768 6.8 Example 37 2682 3162 5.5 Example 382766 3082 5.8 Example 39 2826 3365 5.5 (Note) *Aspect ratio = averagemajor axial diameter/average minor axial diameter

TABLE 5 Kind of low- High-density acicular density acicular Heattreatment for high hematite particles hematite densification AverageAverage particles or Tempera- major axial minor axial Comparativeacicular goethite ture Time diameter diameter Examples particles (° C.)(min) (μm) (μm) Comparative Example 1 720 15 0.075 0.0336 Example 15Comparative Comparative 680 15 0.098 0.0286 Example 16 Example 2Comparative Comparative 700 30 0.125 0.0248 Example 17 Example 4Comparative Comparative 750 60 0.131 0.0220 Example 18 Example 5Comparative Comparative 570 90 0.134 0.0206 Example 19 Example 6Comparative Comparative 720 45 0.134 0.0201 Example 20 Example 7Comparative Comparative 730 30 0.132 0.0218 Example 21 Example 8Comparative Comparative 520 90 0.132 0.0198 Example 22 Example 9Comparative Comparative 720 15 0.191 0.0294 Example 23 Example 10Comparative Comparative 650 40 0.192 0.0288 Example 24 Example 11Comparative Comparative 600 15 0.192 0.0290 Example 25 Example 12Comparative Comparative 750 20 0.190 0.0300 Example 26 Example 13Comparative Comparative 460 60 0.185 0.0292 Example 27 Example 14High-density acicular hematite particles Geometrical Aspect Comparativestandard ratio* S_(BET) S_(TEM) S_(BET)/S_(TEM) Examples deviation σg(−) (−) (m²/g) (m²/g) (−) Comparative 1.84 2.23 15.8 28.0 0.56 Example15 Comparative 1.71 3.43 21.9 30.8 0.71 Example 16 Comparative 1.56 5.0431.8 34.1 0.93 Example 17 Comparative 1.36 5.95 45.6 37.9 1.20 Example18 Comparative 1.35 6.50 59.3 40.2 1.47 Example 19 Comparative 1.35 6.6752.6 41.1 1.28 Example 20 Comparative 1.36 6.06 43.6 38.2 1.14 Example21 Comparative 1.34 6.67 68.9 41.8 1.65 Example 22 Comparative 1.34 6.5035.2 28.2 1.25 Example 23 Comparative 1.33 6.67 43.9 28.7 1.53 Example24 Comparative 1.33 6.62 51.5 28.5 1.81 Example 25 Comparative 1.34 6.3337.5 27.7 1.36 Example 26 Comparative 1.33 6.34 65.0 28.4 2.29 Example27 High-density acicular hematite particles Kind of Sintering preventiveSoluble Soluble Volume Comparative Calculated Amount sodium sulfate pHvalue resistivity Example as (wt. %) salt (ppm) (ppm) (−) (Ωm)Comparative — — 1658 3256 5.3 5.6 × 10⁸ Example 15 Comparative — — 17453569 5.3 8.9 × 10⁸ Example 16 Comparative P 0.44 1652 3756 5.1 3.8 × 10⁸Example 17 Comparative SiO₂ 0.93 1548 3874 4.9 7.1 × 10⁸ Example 18Comparative P 0.46 1436 2964 5.3 — Example 19 Comparative SiO₂ 1.40 15693684 5.1 — Example 20 Comparative SiO₂ 0.21 1856 3548 5.2 — Example 21Comparative P 0.74 1329 2456 5.6 — Example 22 Comparative P 1.83 19543659 5.0 3.8 × 10⁸ Example 23 Comparative SiO₂ 1.22 2045 3246 5.6 —Example 24 Comparative P 1.46 1564 2857 5.2 — Example 25 ComparativeSiO₂ 0.23 1186 3156 4.7 — Example 26 Comparative Sn 0.05 2356 3247 5.6 —Example 27 (Note) *Aspect ratio = average major axial diameter/averageminor axial diameter

TABLE 6 Kind of high- Wet-pulverization Heat treatment density step inaqueous acicular Residue alkaline solution hematite Use or on sieve pHTempera- Time Examples particles non-use (wt. %) (−) ture (° C.) (min)Example 40 Example 22 used 0 13.1 98 180 Example 41 Example 23 used 013.5 94 180 Example 42 Example 24 used 0 13.3 95 180 Example 43 Exampie25 used 0 13.8 91 120 Example 44 Example 26 used 0 13.8 95  90 Example45 Example 27 used 0 13.5 95  90 Example 46 Example 28 used 0 13.6 95180 Example 47 Example 29 used 0 13.5 92 180 Example 48 Example 30 used0 13.7 95 120 Example 49 Example 3i used 0 13.3 90 120 Example 50Example 32 used 0 13.5 97 120 Example 51 Example 33 used 0 13.8 97  60Example 52 Example 34 used 0 13.7 95  60 Example 53 Example 3S used 013.2 90 120 Example 54 Example 36 used 0 13.6 95 180 Example 55 Example37 used 0 13.1 95 180 Example 56 Example 38 used 0 13.5 95 180 Example57 Example 39 used 0 13.3 95 180

TABLE 7 Acicular hematite particles washed with water after aqueousalkaline solution treatment Geometrical Average major Average minorstandard Aspect axial diameter axial diameter deviation ratio* Examples(μm) (μm) σg (−) (−) Example 40 0.132 0.0209 1.35 6.32 Example 41 0.1380.0223 1.38 6.19 Example 42 0.137 0.0230 1.38 5.96 Example 43 0.1700.0301 1.35 5.65 Example 44 0.172 0.0288 1.34 5.97 Example 45 0.1860.0294 1.35 6.33 Example 46 0.203 0.0314 1.34 6.46 Example 47 0.1380.0222 1.39 6.22 Example 48 0.139 0.0243 1.40 5.72 Example 49 0.2100.0298 1.44 7.05 Example 50 0.209 0.0296 1.44 7.06 Example 51 0.1880.0285 1.34 6.60 Example 52 0.189 0.0298 1.35 6.34 Example 53 0.1400.0220 1.36 6.36 Example 54 0.140 0.0219 1.37 6.39 Example 55 0.1410.0213 1.36 6.62 Example 56 0.141 0.0211 1.36 6.68 Example 57 0.1430.0210 1.36 6.81 Acicular hematite particles washed with water afteraqueous alkaline solution treatment S_(BET) S_(TEM) S_(BET)/S_(TEM)Examples (m²/g) (m²/g) (−) Example 40 52.2 39.7 1.31 Example 41 47.137.3 1.26 Example 42 43.7 36.3 1.21 Example 43 38.9 27.8 1.40 Example 4436.1 28.9 1.25 Example 45 34.0 28.2 1.20 Example 46 35.5 26.4 1.35Example 47 50.6 37.4 1.35 Example 48 52.5 34.4 1.53 Example 49 36.9 27.61.33 Example 50 38.3 27.8 1.38 Example 51 40.8 29.0 1.41 Example 52 36.827.8 1.32 Example 53 54.8 37.7 1.45 Example 54 56.5 37.9 1.49 Example 5550.6 38.8 1.30 Example 56 50.8 39.2 1.30 Example 57 51.9 39.3 1.32Acicular hematite particles washed with water after aqueous alkalinesolution treatment Kind of Sintering preventive Cal- Cal- culated Amountculated Amount Calculated Amount Examples as (wt. %) as (wt. %) as (wt.%) Example 40 Sn 14.16 — — — — Example 41 Sn 17.92 — — — — Example 42 Sn36.65 — — SiO₂ 0.60 Example 43 Sn  1.30 — — — — Example 44 Sn  3.14 — —— — Example 45 Sn  9.68 — — — — Example 46 Sn 35.60 — — — — Example 47Sn 72.10 — — — — Example 48 Sn 90.01 — — — — Example 49 Sn 53.65 — — — —Example 50 Sn 45.89 P 0.26 — — Example 51 Sn 35.68 Sb 5.16 — — Example52 Sn  9.26 Sb 1.86 — — Example 53 Sn 48.65 Sb 8.62 SiO₂ 1.36 Example 54Sn 76.56 Sb 13.68  Al 0.98 Example 55 — — P 0.72 — — Example 56 — — — —SiO₂ 1.80 Example 57 — — P 0.25 SiO₂ 0.68 Acicular hematite particleswashed with water after aqueous alkaline solution treatment Solublesodium Volume salt Soluble sulfate pH value resistivity Examples (ppm)(ppm) (−) (Ωcm) Example 40 108 13 9.3 3.2 × 10⁶ Example 41 135 32 9.01.7 × 10⁶ Example 42  87 23 9.1 1.1 × 10⁶ Example 43  78 24 8.8 3.8 ×10⁷ Example 44 121 32 9.4 3.1 × 10⁷ Example 45  98 46 8.6 8.2 × 10⁶Example 46 105 48 8.9 1.0 × 10⁶ Example 47 124 11 9.0 5.8 × 10⁵ Example48 138 21 9.5 2.6 × 10⁵ Example 49  76 15 8.8 9.1 × 105 Example 50  8921 8.9 7.0 × 10⁵ Example 51 107 17 9.3 6.9 × 10⁵ Example 52 124 16 9.12.6 × 10⁶ Example 53  79  8 9.0 3.3 × 10⁵ Example 54  87  8 8.9 1.3 ×10⁵ Example 55 115 15 9.2 8.9 × 10⁸ Example 56 121 21 9.4 6.5 × 10⁸Example 57  89 32 8.9 5.1 × 10⁸ (Note) *Aspect ratio = average majoraxial diameter/average minor axial diameter

TABLE 8 Heat treatment Wet-pulverization in aqueous Kind of stepalkaline solution acicular Residue Tempera- Comparative hematite Use oron sieve pH ture Time Examples particles non-use (wt. %) (−) (° C.)(min) Comparative Comparative used 0 — — — Example 28 Example 19Comparative Comparative used 0 11.5 93 180 Example 29 Example 20Comparative Comparative used 0 13.3 50 180 Example 30 Example 21Comparative Comparative unused 18.0 13.3 90 180 Example 31 Example 22Comparative Comparative unused 19.6 10.5 95 180 Example 32 Example 24Comparative Comparative unused 23.6 13.3 92 120 Example 33 Example 25Comparative Comparative unused 17.5 13.5 90 120 Example 34 Example 26Comparative Comparative used 0  9.5 95 120 Example 35 Example 27

TABLE 9 Acicular hematite particles washed with water after aqueousalkaline solution treatment Average major Average minor GeometricalComparative axial diameter axial diameter standard Aspect ratio*Examples (μm) (μm) deviation σg (−) (−) Comparative 0.134 0.0206 1.356.50 Example 28 Comparative 0.134 0.0200 1.35 6.70 Example 29Comparative 0.132 0.0218 1.35 6.06 Example 30 Comparative 0.132 0.01981.35 6.67 Example 31 Comparative 0.192 0.0288 1.35 6.67 Example 32Comparative 0.192 0.0291 1.34 6.60 Example 33 Comparative 0.191 0.03021.35 6.32 Example 34 Comparative 0.185 0.0292 1.33 6.34 Example 35Acicular hematite particles washed with water after aqueous alkaiinesolution treatrnent Comparative S_(BET) S_(TEM) S_(BET)/S_(TEM) Examples(m²/g) (m²/g) (−) Comparative 58.8 40.2 1.46 Example 28 Comparative 52.641.3 1.27 Example 29 Comparative 44.1 38.2 1.15 Example 30 Comparative69.1 41.8 1.65 Example 31 Comparative 43.7 28.7 1.52 Example 32Comparative 50.9 28.4 1.79 Example 33 Comparative 38.0 27.5 1.38 Example34 Comparative 63.8 28.4 2.24 Example 35 Acicular hematite particleswashed with water after aqueous alkaline solution treatment Kind ofSintering Specific preventive Soluble Soluble volume ComparativeCalculated Amount sodium sulfate pH value resistivity Examples as (wt.%) salt (ppm) (ppm) (−) (Ωcm) Comparative P 0.46 658 354 7.1 5.1 × 10⁸Example 28 Comparative SiO₂ 1.38 452 316 7.0 6.7 × 10⁸ Example 29Comparative SiO₂ 0.21 365 197 7.7 7.4 × 10⁸ Example 30 Comparative P0.75 312 165 7.9 8.0 × 10⁸ Example 31 Comparative SiO₂ 1.20 703 335 7.06.1 × 10⁸ Example 32 Comparative P 1.44 321 185 7.5 7.9 × 10⁸ Example 33Comparative SiO₂ 0.23 376 167 7.9 3.7 × 10⁸ Example 34 Comparative SiO₂0.04 832 349 7.1 8.6 × 10⁸ Example 35 (Note) *Aspect ratio = averagemajor axial diameter/average minor axial diameter

TABLE 10 Kind of acicular Surface treatment hematite Amount particlesadded treated with (calculated aqueous as each Coating substancealkaline element) Calculated Amount Examples solution Kind (wt. %) as(wt. %) Example 58 Example 40 Sodium aluminate 1.3 Al 1.28 Example 59Example 41 Water glass #3 0.5 SiO₂ 0.45 Example 60 Example 42 Aluminumsulfate 1.0 Al 0.99 Example 61 Example 43 Colloidal silica 1.0 SiO₂ 0.96Example 62 Example 44 Aluminum acetate 1.5 Al 1.46 Water glass#3 0.8SiO₂ 0.77 Example 63 Example 45 Aluminum sulfate 0.3 Al 0.30 Water glass#3 2.5 SiO₂ 2.38 Example 64 Example 46 Sodium aluminate 5.0 Al 4.76Example 65 Example 47 Sodium aluminate 1.5 Al 1.46 Colloidal silica 2.5SiO₂ 2.36 Example 66 Example 48 Sodium aluminate 0.2 Al 0.20 Example 67Example 49 Aluminum acetate 10.0  Al 9.01 Colloidal silica 0.3 SiO₂ 0.28Example 68 Example 50 Water glass #3 3.0 SiO₂ 2.90 Example 69 Example 51Sodium aluminate 1.8 Al 1.76 Example 70 Example 52 Aluminum acetate 0.2Al 0.20 Example 71 Example 53 Sodium aluminate 3.0 Al 2.91 Example 72Example 54 Water glass #3 1.5 SiO₂ 1.45 Example 73 Example 55 Sodiumstannate 3.0 Sn 2.90 Example 74 Example 56 Stannous chloride 5.0 Sn 4.71Example 75 Example 57 Sodium stannate 5.5 Sn 5.11 Antimony chloride 0.5Sb 0.47

TABLE 11 Properties of acicular hematite particles washed with waterafter aqueous alkaline solution treatment Geometrical Average majorAverage minor standard Aspect axial diameter axial diameter deviationratio* Examples (μm) (μm) σg (−) (−) Example 58 0.132 0.0209 1.35 6.32Example 59 0.137 0.0222 1.38 6.17 Example 60 0.137 0.0231 1.38 5.93Example 61 0.170 0.0300 1.35 5.67 Example 62 0.171 0.0288 1.35 5.94Example 63 0.187 0.0294 1.35 6.36 Example 64 0.202 0.0313 1.34 6.45Example 65 0.138 0.0222 1.39 6.22 Example 66 0.139 0.0244 1.40 5.70Example 67 0.209 0.0298 1.44 7.01 Example 68 0.210 0.0296 1.44 7.09Example 69 0.188 0.0285 1.34 6.60 Example 70 0.189 0.0298 1.35 6.34Example 71 0.141 0.0221 1.36 6.38 Example 72 0.140 0.0220 1.37 6.36Example 73 0.141 0.0213 1.36 6.62 Example 74 0.141 0.0211 1.36 6.68Example 75 0.143 0.0210 1.36 6.81 Properties of acicular hematiteparticles washed with water after aqueous alkaline solution treatmentS_(BET) S_(TEM) S_(BET)/S_(TEM) Examples (m²/g) (m²/g) (−) Example 5851.9 39.7 1.31 Example 59 46.9 37.5 1.25 Example 60 43.2 36.1 1.20Example 61 38.7 27.9 1.39 Example 62 36.0 29.0 1.24 Example 63 35.3 28.21.25 Example 64 36.5 26.5 1.38 Example 65 52.3 37.4 1.40 Example 66 51.534.3 1.50 Example 67 40.0 27.7 1.45 Example 68 40.8 27.8 1.47 Example 6940.1 29.0 1.38 Example 70 36.5 27.8 1.31 Example 71 55.1 37.5 1.47Example 72 55.6 37.7 1.47 Example 73 51.0 38.8 1.31 Example 74 52.6 39.21.34 Example 75 52.6 39.3 1.34 Properties of acicular hematite particleswashed with water after aqueous alkaline solution treatment Kind ofSintering preventive Cal- Cal- culated Amount culated Amount CalculatedAmount Examples as (wt. %) as (wt. %) as (wt. %) Example 58 Sn 13.75 — —— — Example 59 Sn 17.68 — — — — Example 60 Sn 34.52 — — SiO₂ 0.58Example 61 Sn  1.28 — — — — Example 62 Sn  3.06 — — — — Example 63 Sn 9.22 — — — — Example 64 Sn 33.81 — — — — Example 65 Sn 70.12 — — — —Example 66 Sn 88.82 — — — — Example 67 Sn 48.77 — — — — Example 68 Sn44.00 P 0.25 — — Example 69 Sn 34.68 Sb 5.11 — — Example 70 Sn  8.92 Sb1.83 — — Example 71 Sn 47.32 Sb 8.58 SiO₂ 1.36 Example 72 Sn 74.82 Sb12.99  Al 0.98 Example 73 — — P 0.69 — — Example 74 — — — — SiO₂ 1.71Example 75 — — P 0.23 SiO₂ 0.64 Properties of acicular hematiteparticles washed with water after aqueous alkaline solution treatmentSoluble sodium Volume salt Soluble sulfate pH value resistivity Examples(ppm) (ppm) (−) (Ωcm) Example 58 97  9 9.3 4.1 × 10⁶ Example 59 76 139.0 1.8 × 10⁶ Example 60 65  6 9.4 1.3 × 10⁶ Example 61 85 12 8.9 5.0 ×10⁷ Example 62 56  2 9.6 4.5 × 10⁷ Example 63 123  12 9.3 1.2 × 10⁷Example 64 76 10 9.0 2.3 × 10⁶ Example 65 107   7 9.2 6.9 × 10⁶ Example66 65  5 9.6 2.5 × 10⁶ Example 67 135  34 8.8 2.4 × 10⁶ Example 68 87 129.1 9.3 × 10⁵ Example 69 54 11 9.0 8.8 × 10⁵ Example 70 46  2 9.2 2.7 ×10⁶ Example 71 68 16 9.0 6.4 × 10⁵ Example 72 100  11 8.9 2.5 × 10⁵Example 73 89 12 9.2 6.8 × 10⁸ Example 74 78  6 9.3 3.2 × 10⁸ Example 7566 11 9.0 1.1 × 10⁸ (Note) *Aspect ratio = average major axialdiameter/average minor axial diameter

TABLE 12 Kind of high- density acicular Heat treatment Coating substancehematite Temperature Time Calculated Amount Examples particles (° C.)(min) as (wt. %) Example 76 Example 73 400 60 Sn 2.92 Example 77 Example74 350 60 Sn 4.75 Example 78 Example 75 380 90 Sn 5.32 Sb 0.49

TABLE 13 Properties of acicular hematite particles washed with waterafter treatment with tin compound Geometrical Average major Averageminor standard Aspect axial diameter axial diameter deviation ratio*Examples (μm) (μm) σg (−) (−) Example 76 0.141 0.0213 1.36 6.62 Example77 0.141 0.0210 1.36 6.71 Example 78 0.143 0.0209 1.36 6.84 Propertiesof acicular hematite particles washed with water after treatment withtin compound S_(BET) S_(TEM) S_(BET)/S_(TEM) Examples (m²/g) (m²/g) (−)Example 76 50.6 38.8 1.30 Example 77 51.1 39.4 1.30 Example 78 51.9 39.51.31 Properties of acicular hematite particles washed with water aftertreatment with tin compound Kind of Sintering preventive ExamplesCalculated as Amount (wt. %) Example 76 P 0.70 Example 77 SiO₂ 1.74Example 78 P 0.24 SiO₂ 0.67 Properties of acicular hematite particleswashed with water after treatment with tin compound Soluble sodiumVolume salt Soluble sulfate pH value resistivity Examples (ppm) (ppm)(−) (Ωcm) Example 76 125 15 8.9 2.6 × 10⁶ Example 77 115 26 9.0 8.1 ×10⁶ Example 78  91 32 9.0 1.6 × 10⁶ (Note) *Aspect ratio = average majoraxial diameter/average minor axial diameter

TABLE 14 Production of non-magnetic coating material Non-magnetic Kindof coating acicular Weight ratio material hematite of particlesViscosity Examples particles to resin (−) (cP) Example 79 Example 40 5.0410 Example 80 Example 41 5.0 435 Example 81 Example 42 5.0 384 Example82 Example 43 5.0 333 Example 83 Example 44 5.0 205 Example 84 Example45 5.0 179 Example 85 Example 46 5.0 128 Example 86 Example 47 5.0 742Example 87 Example 48 5.0 896 Example 88 Example 49 5.0 205 Example 89Example 50 5.0 230 Example 90 Example 51 5.0 512 Example 91 Example 525.0 384 Example 92 Example 53 5.0 768 Example 93 Example 54 5.0 819Example 94 Example 58 5.0 384 Example 95 Example 59 5.0 384 Non-magneticundercoat layer Young's Thickness Surface modulus of coating Glossroughness (relative Examples layer (μm) (%) Ra (nm) value) Example 793.5 190 7.2 126 Example 80 3.4 186 7.5 124 Example 81 3.4 181 7.8 125Example 82 3.4 198 6.8 129 Example 83 3.3 196 6.8 127 Example 84 3.5 1947.6 133 Example 85 3.5 188 8.4 131 Example 86 3.4 185 8.8 124 Example 873.3 180 10.4 123 Example 88 3.5 187 8.0 134 Example 89 3.4 191 7.8 136Example 90 3.5 195 7.4 128 Example 91 3.4 199 7.0 131 Example 92 3.5 1858.6 124 Example 93 3.5 185 8.3 126 Example 94 3.4 185 7.0 128 Example 953.5 180 7.3 125

TABLE 15 Production of non-magnetic coating material Non-magnetic Kindof coating acicular Weight ratio material hematite of particlesViscosity Examples particles to resin (−) (cP) Example 96  Example 605.0 307 Example 97  Example 61 5.0 333 Example 98  Example 62 5.0 179Example 99  Example 63 5.0 128 Example 100 Example 64 5.0 102 Example101 Example 65 5.0 512 Example 102 Example 66 5.0 768 Example 103Example 67 5.0 179 Example 104 Example 68 5.0 205 Example 105 Example 695.0 384 Example 106 Example 70 5.0 230 Example 107 Example 71 5.0 410Example 108 Example 72 5.0 435 Example 109 Example 76 5.0 384 Example110 Example 77 5.0 435 Example 111 Example 78 5.0 512 Non-magneticundercoat layer Young's Thickness Surface modulus of coating Glossroughness (relative Examples layer (μm) (%) Ra (nm) value) Example 96 3.4 191 7.3 127 Example 97  3.4 205 6.8 131 Example 98  3.4 200 6.5 131Example 99  3.3 193 7.4 136 Example 100 3.4 195 7.9 134 Example 101 3.4190 8.3 125 Example 102 3.5 188 8.6 125 Example 103 3.5 193 8.8 136Example 104 3.4 194 8.5 139 Example 105 3.5 196 8.0 131 Example 106 3.4207 6.8 135 Example 107 3.3 190 7.4 127 Example 108 3.4 195 7.3 127Example 109 3.4 195 7.5 128 Example 110 3.4 198 7.2 128 Example 111 3.4201 6.9 129

TABLE 16 Production of non-magnetic coating material Non-magnetic Kindof coating acicular Weight ratio material Comparative hematite ofparticles Viscosity Examples particles to resin (−) (cP) ComparativeComparative 5.0 25600 Example 36 Example 1 Comparative Comparative 5.0128 Example 37 Example 15 Comparative Comparative 5.0 102 Example 38Example 16 Comparative Comparative 5.0 20480 Example 39 Example 3Comparative Comparative 5.0 768 Example 40 Example 17 ComparativeComparative 5.0 563 Example 41 Example 18 Comparative Comparative 5.0435 Example 42 Example 28 Comparative Comparative 5.0 405 Example 43Example 29 Comparative Comparative 5.0 384 Example 44 Example 30Comparative Comparative 5.0 640 Example 45 Example 31 ComparativeComparative 5.0 512 Example 46 Example 23 Comparative Comparative 5.0435 Example 47 Example 32 Comparative Comparative 5.0 435 Example 48Example 33 Comparative Comparative 5.0 410 Example 49 Example 34Comparative Comparative 5.0 2560 Example 50 Example 35 Non-magneticundercoat layer Young's Thickness Surface modulus Comparative of coatingGloss roughness (relative Examples layer (μm) (%) Ra (nm) value)Comparative 3.6 68 112.0 76 Example 36 Comparative 3.5 45 181.0 58Example 37 Comparative 3.5 89 78.5 86 Example 38 Comparative 3.7 76 84.077 Example 39 Comparative 3.5 135 33.8 103 Example 40 Comparative 3.5159 23.1 105 Example 41 Comparative 3.5 170 15.9 110 Example 42Comparative 3.6 176 13.1 112 Example 43 Comparative 3.5 175 13.2 110Example 44 Comparative 3.6 165 17.0 105 Example 45 Comparative 3.7 14721.7 109 Example 46 Comparative 3.6 167 18.6 112 Example 47 Comparative3.8 171 16.2 110 Example 48 Comparative 3.5 175 14.4 114 Example 49Comparative 3.6 148 22.7 104 Example 50

TABLE 17 Magnetic recording medium using magnetic particles containingiron as main ingredient Weight ratio Kind of of mag- Non- netic magneticparticles undercoat Kind of magnetic particles to resin Examples layercontaining iron as main ingredient (−) Example 112 Example 79 [majoraxial diameter = 0.10 μm; 5.0 minor axial diameter = 0.016 μm; Example113 Example 80 aspect ratio = 6.3; 5.0 Example 114 Example 81 Hc = 1926; 5.0 Example 115 Example 82 δs = 131.0 emu/g; 5.0 Example 116 Example83 pH value = 10.3; 5.0 Example 117 Example 84 Al content = 4.11 wt. %;5.0 Example 118 Example 85 Co content = 5.87 wt. %] 5.0 Example 119Example 86 [major axial diameter = 0.12 μm; 5.0 Example 120 Example 87minor axial diameter = 0.018 μm; 5.0 Example 121 Example 88 aspect ratio= 7.0; 5.0 Example 122 Example 89 Hc = 1770 ; 5.0 Example 123 Example 90δs = 138.0 emu/g; 5.0 Example 124 Example 91 pH value = 9.8; 5.0 Example125 Example 92 Al content = 2.27 wt. %; 5.0 Example 126 Example 93 Cocontent = 3.72 wt. %] 5.0 Magnetic recording medium using magneticparticles containing iron as main ingredient Thickness of CoerciveSurface magnetic force Br/Bm Gloss roughness Examples layer (μm) () (−)(%) Ra (nm) Example 112 1.1 2022 0.88 210 7.0 Example 113 1.1 2035 0.88206 7.0 Example 114 1.1 2041 0.88 205 7.6 Example 115 1.0 2087 0.88 2216.4 Example 116 1.1 2036 0.88 223 6.3 Example 117 1.1 2026 0.87 212 7.0Example 118 1.1 2016 0.88 208 7.4 Example 119 1.1 1856 0.89 203 8.0Example 120 1.2 1836 0.87 196 8.6 Example 121 1.1 1897 0.88 200 7.6Example 122 1.0 1893 0.89 211 7.4 Example 123 1.1 1834 0.89 209 7.2Example 124 1.1 1867 0.90 227 6.5 Example 125 1.2 1870 0.88 213 7.0Example 126 1.1 1867 0.88 216 7.0 Magnetic recording medium usingmagnetic particles containing iron as main ingredient Corrosion propertyYoung's Linear Rate of modulus absorption Surface change in Rate of(relative coefficient resistivity coercive change in Examples value)(μm⁻¹) (Ω/sq) force (%) Bm (%) Example 112 131 1.23 9.6 × 10⁷ 7.9 6.9Example 113 130 1.24 5.1 × 10⁷ 6.4 5.4 Example 114 132 1.21 9.8 × 10⁷5.3 5.8 Example 115 134 1.27 4.6 × 10⁷ 5.7 6.9 Example 116 134 1.29 4.4× 10⁷ 7.3 6.4 Example 117 138 1.31 1.8 × 10⁸ 3.7 4.8 Example 118 1351.31 8.8 × 10⁷ 6.4 7.9 Example 119 130 1.20 6.4 × 10⁶ 4.8 6.3 Example120 131 1.20 7.8 × 10⁵ 5.3 7.2 Example 121 139 1.30 8.6 × 10⁶ 8.6 7.6Example 122 141 1.31 7.3 × 10⁶ 7.4 7.9 Example 123 132 1.28 1.0 × 10⁷5.8 6.8 Example 124 133 1.26 7.8 × 10⁷ 6.7 5.2 Example 125 130 1.23 6.5× 10⁶ 6.6 4.8 Example 126 130 1.22 8.7 × 10⁵ 3.6 3.5

TABLE 18 Magnetic recording medium using magnetic particles containingiron as main ingredient Weight ratio Kind of of mag- Non- netic magneticparticles undercoat Kind of magnetic particles to resin Examples layercontaining iron as main ingredient (−) Example 127 Example 94 [majoraxial diameter = 0.10 μm; 5.0 minor axial diameter = 0.016 μm; Example128 Example 95  aspect ratio = 6.3; 5.0 Example 129 Example 96  Hc =1926 ; 5.0 Example 130 Example 97  δs = 131.0 emu/g; 5.0 Example 131Example 98  pH value = 10.3; 5.0 Example 132 Example 99  Al content =4.11 wt. %; 5.0 Example 133 Example 100 Co content = 5.87 wt. %] 5.0Example 134 Example 101 [major axial diameter = 0.12 μm; 5.0 Example 135Example 102 minor axial diameter = 0.018 μm; 5.0 Example 136 Example 103aspect ratio = 7.0; 5.0 Example 137 Example 104 Hc = 1770 ; 5.0 Example138 Example 105 δs = 138.0 emu/g; 5.0 Example 139 Example 106 pH value =9.8; 5.0 Example 140 Example 107 Al content = 2.27 wt. %; 5.0 Example141 Example 108 Co content = 3.72 wt. %] 5.0 Example 142 Example 109 5.0Example 143 Example 110 5.0 Example 144 Example 111 5.0 Magneticrecording medium using magnetic particles containing iron as mainingredient Thickness of Coercive Surface magnetic force Br/Bm Glossroughness Examples layer (μm) () (−) (%) Ra (nm) Example 127 1.1 20320.89 223 6.8 Example 128 1.1 2043 0.88 215 7.0 Example 129 1.1 2036 0.89209 7.3 Example 130 1.0 2075 0.88 220 6.4 Example 131 1.1 2046 0.90 2256.2 Example 132 1.1 2050 0.89 227 6.8 Example 133 1.2 2043 0.90 219 7.2Example 134 1.2 1864 0.90 218 7.6 Example 135 1.2 1845 0.88 209 9.0Example 136 1.1 1887 0.88 210 7.4 Example 137 1.1 1895 0.90 217 7.0Example 138 1.0 1845 0.90 217 7.0 Example 139 1.1 1876 0.91 236 6.1Example 140 1.1 1881 0.88 218 6.8 Example 141 1.1 1875 0.89 223 6.7Example 142 1.2 1881 0.89 211 7.5 Example 143 1.1 1872 0.90 215 7.3Example 144 1.1 1865 0.90 216 7.0 Magnetic recording medium usingmagnetic particles containing iron as main ingredient Corrosion propertyYoung's Linear Rate of modulus absorption Surface change in Rate of(relative coefficient resistivity coercive change in Examples value)(μm⁻¹) (Ω/sq) force (%) Bm (%) Example 127 134 1.24 9.6 × 10⁷ 4.5 4.7Example 128 130 1.24 5.3 × 10⁷ 5.4 4.9 Example 129 133 1.23 1.0 × 10⁸4.5 4.7 Example 130 136 1.27 4.6 × 10⁸ 5.5 6.6 Example 131 137 1.29 4.8× 10⁸ 6.8 6.3 Example 132 141 1.32 2.5 × 10⁸ 3.2 3.8 Example 133 1381.31 9.9 × 10⁷ 3.6 4.7 Example 134 130 1.21 7.2 × 10⁶ 3.7 5.1 Example135 131 1.21 8.5 × 10⁵ 5.0 6.4 Example 136 139 1.30 9.0 × 10⁶ 6.8 6.5Example 137 144 1.30 8.7 × 10⁶ 6.1 6.0 Example 138 132 1.28 2.1 × 10⁷4.7 5.5 Example 139 140 1.27 8.4 × 10⁷ 5.4 4.1 Example 140 134 1.23 8.0× 10⁶ 2.9 3.7 Example 141 135 1.23 9.8 × 10⁵ 2.8 2.9 Example 142 1331.23 3.6 × 10⁸ 7.2 6.8 Example 143 134 1.23 2.1 × 10⁸ 8.1 6.8 Example144 135 1.24 9.6 × 10⁷ 6.0 5.2

TABLE 19 Magnetic recording medium using magnetic particles containingiron as main ingredient Weight ratio Kind of of mag- Non- netic magneticparticles Comparative undercoat Kind of magnetic particles to resinExamples layer containing iron as main ingredient (−) ComparativeComparative [major axial diameter = 0.10 μm; 5.0 Example 51 Example 36minor axial diameter = 0.016 μm; Comparative Comparative aspect ratio =6.3; 5.0 Example 52 Example 37 Hc = 1926 ; Comparative Comparative δs =131.0 emu/g; 5.0 Example 53 Example 38 pH value = 10.3; ComparativeComparative Al content = 4.11 wt. %; 5.0 Example 54 Example 39 Cocontent = 5.87 wt. %] Comparative Comparative 5.0 Example 55 Example 40Comparative Comparative 5.0 Example 56 Example 41 ComparativeComparative 5.0 Example 57 Example 42 Comparative Comparative 5.0Example 58 Example 43 Comparative Comparative 5.0 Example 59 Example 44Comparative Comparative 5.0 Example 60 Example 45 ComparativeComparative 5.0 Example 61 Example 46 Comparative Comparative 5.0Example 62 Example 47 Comparative Comparative 5.0 Example 63 Example 48Comparative Comparative 5.0 Example 64 Example 49 ComparativeComparative 5.0 Example 65 Example 50 Magnetic recording medium usingmagnetic particles containing iron as main ingredient Thickness ofCoercive Surface Comparative magnetic force Br/Bm Gloss roughnessExamples layer (μm) () (−) (%) Ra (nm) Comparative 1.3 1976 0.77 12376.5 Example 51 Comparative 1.2 1987 0.81 132 68.3 Example 52Comparative 1.2 1980 0.82 165 31.6 Example 53 Comparative 1.2 1991 0.78154 46.2 Example 54 Comparative 1.2 2001 0.83 175 17.9 Example 55Comparative 1.3 2010 0.84 187 13.2 Example 56 Comparative 1.1 2018 0.86191 11.8 Example 57 Comparative 1.3 2028 0.87 194 11.6 Example 58Comparative 1.1 1999 0.86 190 12.6 Example 59 Comparative 1.2 2007 0.84177 14.7 Example 60 Comparative 1.1 1989 0.84 165 21.6 Example 61Comparative 1.0 2011 0.85 181 13.8 Example 62 Comparative 1.3 1997 0.85188 12.1 Example 63 Comparative 1.3 2023 0.85 188 11.9 Example 64Comparative 1.2 2017 0.84 165 23.8 Example 65 Magnetic recording mediumusing magnetic particles containing iron as main ingredient Corrosionproperty Young's Linear Rate of modulus absorption Surface change inRate of Comparative (relative coefficient resistivity coercive change inExamples value) (μm⁻¹) (Ω/sq) force (%) Bm (%) Comparative  90 0.84 8.9× 10⁸ 27.5 25.4 Example 51 Comparative  73 0.90 8.3 × 10⁸ 38.9 31.1Example 52 Comparative  96 0.95 9.6 × 10⁸ 49.8 36.8 Example 53Comparative  89 0.99 1.1 × 10⁹ 28.2 23.7 Example 54 Comparative 113 1.091.3 × 10⁹ 46.9 39.7 Example 55 Comparative 116 1.13 8.7 × 10⁸ 37.6 33.3Example 56 Comparative 121 1.15 2.3 × 10⁹ 17.1 15.8 Example 57Comparative 116 1.15 1.0 × 10¹⁰ 14.2 13.8 Example 58 Comparative 1211.14 7.2 × 10⁹ 16.4 16.7 Example 59 Comparative 121 1.17 3.6 × 10⁹ 16.319.0 Example 60 Comparative 116 1.19 1.8 × 10⁹ 37.4 31.6 Example 61Comparative 121 1.19 4.1 × 10⁹ 18.9 23.1 Example 62 Comparative 119 1.177.2 × 10⁹ 15.7 18.5 Example 63 Comparative 123 1.18 6.5 × 10⁹ 18.5 16.9Example 64 Comparative 118 1.06 9.6 × 10⁸ 23.7 24.8 Example 65

What is claimed is:
 1. A magnetic recording medium comprising: anon-magnetic substrate; a non-magnetic undercoat layer formed on saidmagnetic substrate comprising high-density acicular hematite particlesand a coat comprising an oxide of tin or oxides of tin and antimony,formed on at least a part of surfaces of said acicular hematiteparticles, said particles having an average major axial diameter of notmore than 0.3 μm, a pH value of not less than 8, a soluble sodium saltcontent of not more than 300 ppm, calculated as Na, a soluble sulfatecontent of not more than 150 ppm, calculated as SO4, and a volumeresistivity of 10³ to 5×10⁷ Ωcm and a binder resin; and a magneticrecording layer comprising magnetic particles containing iron as a mainingredient and a binder resin, formed on said non-magnetic undercoatlayer.
 2. A magnetic recording medium according to claim 1, wherein saidmagnetic particles containing iron as a main ingredient comprise 50 to99% by weight of iron, 0.05 to 10% by weight of aluminum, and at leastone selected from the group consisting of Co, Ni, P, Si, Zn, Ti, Cu, B,Nd, La and Y.
 3. A magnetic recording medium according to claim 1,wherein said magnetic particles containing iron as a main ingredientcomprise 50 to 99% by weight of iron, 0.05 to 10% by weight of aluminum,and at least one rare earth metal selected from the group consisting ofNd, La and Y.
 4. A magnetic recording medium according to claim 1,wherein said magnetic particles containing iron as a main ingredienthave an average major axial diameter of 0.01 to 0.50 μm, an averageminor axial diameter of 0.0007 to 0.17 μm, an aspect ratio of not lessthan 3:1, a resin adsorptivity of not less than 65%, a coercive force of1200 to 3200 Oe, and a saturation magnetization of 100 to 170 emu/g. 5.A magnetic recording medium according to claim 1, which further have ancoercive force of 900 to 3500 Oe, a squareness of 0.85 to 0.95, a glossof 195 to 300%, a surface roughness of not more than 11.0 nm, a linearadsorption coefficient of 1.10 to 2.00 μm⁻¹, a surface resistivity of10⁴ to 5×10⁸ Ω/sq.
 6. A magnetic recording medium according to claim 1,which further have a percentage of change in said coercive force of notmore than 10.0%, and a percentage of change in said saturationmagnetization flux of not more than 10.0%.
 7. A magnetic recordingmedium according to claim 1, wherein said high-density acicular hematiteparticles further comprise a coat comprising at least one compoundselected from the group consisting of a hydroxide of aluminum, an oxideof aluminum, a hydroxide of silicon and an oxide of silicon.
 8. Amagnetic recording medium according to claim 7, wherein the amount ofsaid hydroxide of aluminum or said oxide of aluminum contained in saidcoat is 0.01 to 50% by weight, calculated as Al, based of the weight ofsaid acicular hematite particles.
 9. A magnetic recording mediumaccording to claim 7, wherein the amount of said hydroxide of silicon orsaid oxide of silicon contained in said coat is 0.01 to 50% by weight,calculated as SiO₂, based of the weight of said acicular hematiteparticles.
 10. A magnetic recording medium according to claim 1, whereinthe amount of said oxide of tin is 0.5 to 500% by weight, calculated asSn, based on the weight of said acicular hematite particles.
 11. Amagnetic recording medium according to claim 1, wherein the amount ofsaid oxide of antimony when present is 0.05 to 50% by weight, calculatedas Sb, based of the weight of said acicular hematite particles.
 12. Amagnetic recording medium according to claim 1, wherein both Sn and Sbare present in a weight ratio of tin to antimony of 20:1 to 1:1.
 13. Amagnetic recording medium according to claim 1, wherein saidhigh-density acicular hematite particles have an aspect ratio (averagemajor axial diameter:average minor axial diameter) of not less than 2:1.14. A magnetic recording medium according to claim 1, wherein saidhigh-density acicular hematite particles have a degree of densification(S_(BET)/S_(TEM)) of 0.5 to 2.5, wherein S_(BET) represents a specificsurface area measured by a BET method, and S_(TEM) represents a surfacearea calculated from values of major axial diameter and minor axialdiameter obtained by measurement of particles on electron microscopephotograph.
 15. A magnetic recording medium according to claim 1,wherein said high-density acicular hematite particles have a particlesize distribution of major axial diameter represented by a geometricalstandard deviation of not more than 1.50.
 16. A magnetic recordingmedium according to claim 1, wherein said high-density acicular hematiteparticles have a BET specific surface area of not less than 35 m²/g.